AIBU to not want my brother living in my flat long term

Thing is with ground rules in such circs, they are of no concern to anyone whose intention it is to exploit a situation where their ‘victim’ will be seen by those with the financial and cultural power (ma and pa) as on the back foot and in the wrong as a matter of course. Ground rules are trodden underfoot.
I think OP has to defy tradition, but with all appearance of rationality and calmness; not combative (yet), and refuse to accede to this arrangement.

Her peace of mind, her work, her health, her studying perhaps, and above all her precious independence, are under threat and she is old enough to insist they not be tossed aside now.
Who knows, the brother could settle in and start feeding ma and pa tales of sis’s Westernised debauchery, her scofflaw friends and dates even; her eating habits, her mini skirt!
Drip-feeding such poison could have him as good as owning the flat before she can say Jack Robinson, unless she activates her titanium spine and determined jaw.

It sounds like you’re saying that your Brother would judge the way that you live, as an adult, in your own home, would not approve and would potentially talk to your parents about it.

For example, it could be something as simple as liking a glass of wine with dinner and your parents not approving of alcohol consumption, or it could be that you want to have adult intimate relationships (perfectly normal) and that your brother/parents wouldn’t approve.

It could be that you’re in a same sex relationship and your brother/parents are homophobes.

Or it could be something as simple as enjoying Doritos and Cake for dinner some nights, when your parents would expect you to eat a home cooked meal, and provide one for your brother.

All of these, and every other thing that you do in your own home is your business and not theirs. Whilst the money they gave you muddied the waters, it was a gift and they didn’t tell you that it was to be accepted upon you agreeing to have various family members to stay with you whenever they wanted.

You’re an adult now, if your parents disapprove of you not letting your brother live with you, is that any worse than how they would disapprove of the way you love your life if your brother DOES live with you and reports your every move back to them?

I think, given the cultural background you depict, that you have all the more reason to refuse accommodation to your brother. You are fighting from a gender disadvantage already, do you need to be doubly resolute.
I can foresee a day when your parents tell you the flat must be handed over to him, as his are the more important needs, and you return to the family home!
Males will always be more precious and your brother could probably convince your mum about all kinds of selfish plans. He does sound most manipulative, and I’d be feeling very wary indeed.

I was thinking of this overnight, as I’d freak out at the thought of unwanted guests at the best of times.
I think, on the matter of your having received financial help from them, you absolutely must not weaken there. You don’t owe your parents just anything they demand, whatever the ££ circs. There has to be a limit, or that financial help is actually shackling you to them, which rather defeats the object of leaving home.

If I were you therefore, and if this issue were actually a talking point when you (I HOPE!) tell them it’s a ‘No way’ from you, I’d calmly offer to repay that deposit in monthly instalments rather than have your right to privacy and solitude etc depend on their goodwill. Say it as though this is a commonplace suggestion, no big deal. (Though fingers x’d they don’t call your bluff!)

Main thing: guard your independence like a holy relic.
If your brother gets his feet under your table you’ll end up like the recent poster who took YEARS to wrest back her own home from an unscrupulous, manipulative brother-as-lodger. It was a horror story.

I would suck it up, good boundary’s from the off, don’t mother him, it’s not forever

It might improve your relationship if you did ? Idk 🤷‍♀️
I probably would agree but have some ground rules in place before he moves in.

Ooh, I love it!

Could you turnaround your culture on to your parents a bit?

Tell them that you have girlfriends come to stay a lot and they wouldn’t be comfortable with a male in the house?

Hard no - you'll end up massively falling out. I nearly shared with a sibling, it fell through which with hindsight was a good thing - we'd probably have been estranged within 48 hours!

He doesn’t want to flatshare with strangers but also doesn’t want to spend so much on rent for 1 bed place in zone 1 or 2. He also earns much more than me

He may not want those options but they are his options and it sounds like he can afford it. Most people who move to London end up in a flat share or paying extortionate rent. That’s what you get in London. He could live somewhere cheaper and commute? But ultimately he shouldn’t get to impose on you in order to swerve what most people including yourself have to go through. He sounds entitled.

If I were you I’d find a way to say no. Once he is in it’ll be harder to get him out.

You will bitterly regret saying yes.

Given your parents are also helping him with the deposit, they don’t have much leverage here.

Would he let you stay with him for 10 months?

As someone with pushy family members, I tend to be creative with the truth.

perhaps you’ve had your hours/pay cut at work and have taken in a female lodger at market rate. Ask your brother if he can pay the same amount which he won’t want to. Job doneeeee

As a parent, I hope to help at least one of my children with a house deposit, if not both. (One of them is in a better paid profession.) I wouldn't think that just because I helped one with the deposit, he was compelled to house his brother (although my children are far closer than you and your brother seem to be). I think your parents are being very unfair especially as your brother earns more than you do. It's not as if he couldn't pay for a a 1 bedroom place in a nice area, it's just that he doesn't want to. It would be entirely different if he was facing eviction and homelessness but he is not. You deserve to enjoy your twenties - your brother undoubtedly did. This is the time when you might expect to meet the person you eventually marry or partner up with. You don't need your brother as a freeloading gooseberry in your flat.

Charge him full market rent.
You'll lose your single adult discount on council tax anyway so it will go some way to mitigating that.

@youngersibling when money is gifted for a UK property purchase the source of funds anti money laundering process requires written confirmation that it is a gift without reservation. So your parents can't guilt trip you into this.
However, the Monday to Thursday suggestion sounds pragmatic for family harmony for 6 months or until his property complete, whichever is sooner. There's plenty of empty London flats on the market at the moment and prices have fallen massively. Very much a buyer's market.
Charge half market rent, pay for a cleaner out of it if needed and you get the flat to yourself for 3 evenings and all weekend.

The money your parents gave you towards your deposit is obviously giving them a feeling of ownership over your home. Its horrible when you realise that a gift comes with strings attached but here we are.

I'd probably be trying to pay them back if that's do-able. If it isn't, I think you need to tell them that you took their help on face value and you won't allow them to start adding new terms to the deal after the fact. You've been reasonable offering a few months and between them they're pushing for almost a year and you have no control over whether it goes beyond that or not. If you were to get to 10 months and it transpires that he hasn't saved enough or he can't get a mortgage or something you're probably going to face the same pressure and guilt tripping when you try to get him to leave. You may as well deal with it now.

I’m with you. I’m not getting why this is such a big deal unless there’s more to it?

This is one of those BIG FOUR decisions you get in life. If you choose route ‘A’ then ‘x’ happens if you choose ‘B’ it takes you down a completely different path. You don’t realise how pivotal it was until you are in your 40s and you look back on your 20s and 30s and say to yourself - why didn’t my career take off or why did I not meet the right person or how did I end up with such an amazing life?
I bet everyone on this thread is thinking DON’T DO it because it will restrict your way of life.
As someone who has just given a DC miney to help their deposit I get what you think but don’t let bully brother ruin your twenties and the freedom which is your right of passage. Sending assertiveness and resolve to you, OP.

Stop talking to them about it. You and your brother are adults. I would literally say ‘I’m not talking to you about it, nor am I going to be guilt tripped into not doing what I want with my own property. Brother and I will sort it out between us’.

If you’re not able to have robust boundaries with your own brother and say ‘you can only stay 4 months and this is the cost’ then DO NOT allow him to move in.

I fear you are going to have to pull your big girls pants on my dear. You are under no obligation to house your brother. Let him get a house share as you did for so many years. Why is he less capable than you of doing so?

I agree with many other posters and would 100% say no. I have lived independently of my parents since the 1960s and could not share a space especially with a sibling. I once got cajoled into housing a friend for a couple of nights when she had a row with their DH. I was glad when she left. Next time she arrived at my doorstep with an overnight bag I saw her through the keyhole (no ring doorbells back then) and did not answer the door, She sat outside for 2 hours before finally decamping.

@youngersibling name your price.

If you were contemplating a lodger who would have paid say £1000 a month then tell your parents you were contemplating getting a lodger so if your brother moves in he will need to pay the going lodger rate.

Put the £1000 aside and then keep saving until you have enough to pay back the deposit to your parents.

And dont forget that nice long list of lodger rules...

He might hate living with you and move out sooner.

It will probably be longer than six months, probably not any failing or unreasonableness from your brother. Whatever you decide take that into account.