DS girlfriend at our house all the time.

@Mayflowerz has a point, but I think a complete ban would probably just add fuel to the infatuation phase as another OP put it. Just tell him that there will be times when she is not welcome and if he doesn't keep to that then you will tell her that by not keeping to it he has put you all in the awkward position of having to tell her to leave immediately. And then do it. Also tell him that you will be telling her to go home every time she wanders into your office (or anyone else's private space) and every time you hear her bullying your younger son.

Once he knows you will follow through he is likely to adjust if only to avoid embarrassment.

Sounds like she’s trying to move in. You need to have firm boundaries or she’ll be permanent before long.

If your son is ignoring you then tell her directly she can come on X & Y day from A to C time. If they don't stick to it then a complete ban.

I’ve not voted because although I wouldn’t like this, you also seem to not have strong boundaries in place.

Make a new rule that no one can have people around Monday-Thursday.

If he wants to see her then they can go out somewhere or go to her parents place.

You need to put your foot down with both of them. Either they respect your home and your boundaries or they can both get out. Can your DS not go round to her house for a change?

I'd have firmly told her to leave before the family meeting re putting your dog down.

I'd have blown my lid over this already. I like my privacy and to be able to relax in my own home, I cannot relax when visitors are here.

I think OP you need to be very firm with this, directly to this girl, as clearly your son is not respecting your boundaries.

You are allowing your son to assert his ‘male authority’. You need to stamp it out sharpish. Where is your DH in all of this, doesn’t sound as if he’s backing you up. She’d only come into my office once, no need for aggression or shouting, just clear boundaries showing whose house it is and who’s in charge.

Is she working/studying, OP?

Why on earth are you not asserting yourself (politely) with a 16-year-old girl?!

I am not ashamed to admit that I used to shout, "Everybody OUT! Haven't you got homes to go to?" when I'd had enough of the boyfriends/girlfriends. Mind you I used to work in a pub and became immune to throwing people out.

Used to close the door to the sitting room too and shoot the evils at any intrusion. They could go and sit in the bloody kitchen or bugger off home.

I can see both sides tbh. She is at her boyfriends house where he lives. As a person who lives there he has said she’s welcome there.

However, you are the hierarchical leader of the home who gets to say who comes and goes. And you don’t want her there.

The answer is to tell your son she must limit her time there.

i couldn’t put up with this, OP
Particularly with regard to your family losing your Ddog.
There’s only me and DP here and our lovely Ddog passed last June. The tears and snotty noses were plenty but it didn’t matter as there was only us 2 here. We cried together privately which is as it should be.
Out of good manners, she should have left you to grieve as a family. I honestly would say something, it’s just too much.
My condolences of the loss of your dog. 🌈

It's your home OP and she's going impacting on your privacy.

If your DS won't listen then you tell her. No doubt he tells her it's fine. Ask her not to come in to the office or tell her when it's time to leave. She can't expect to be treated like a guest when she's acting like she lives there.

She’s allowed around two evenings a week only on days you agree to. Does not go wandering around the house or bossing your younger son. I’d already have told her to leave but I’m not good with strangers in my home.

Omg has she no actual home to go to ?! This is such an invasion of your privacy!! Very strange that she’s spending so much time in your home.
Lay down some rules, tell your son she can visit 1-2 evenings a week only. and she has to be kind to your poor 16 year old that’s outrageous him being bossed in his own home.

I have four now grown kids youngest 22 and just finishing uni, between the four obviously their bf/gf have come and gone , but unless it was a weekend they would not be around our home. Perhaps old fashioned but Sunday to Thursday is either working/ studying or doing things ready for work or winding down after work.
If your son doesn't like it he needs to save up and move out, obviously difficult at 21.
I also hate cooking for non family members( as am bit rubbish) so if they had ppl over at weekend they have to buy and cook their own food.

Ask her for board. She'll leave sharpish.

Unfortunately, they are in that infatuation stage but you need to put boundaries in place.

Yeah, you’re being too passive and the message has not been received so you need to be more blunt.

Start giving her chores. Say you seem to live here so please crack on with this huge pile of ironing then clean the bathroom

Agree. They're taking the piss.

You need to ban her altogether then. If your son can’t respect your rules then she is not allowed in until he can.

You are being too nice and letting him get away with it. When you arrived home you should have called them down and asked them to leave. It’s your home and they are showing you no respect.

You have to be crystal clear with your son in very plain language. This is his home, but it's not his house. His girlfriend is a guest, who comes at invitation, respects boundaries, and leaves.

If they want to play house together, then he goes and gets his own place.