If you saw a bridesmaid crying ugly tears (not necessarily happy) what would you think?

I’m not sure why people are being so knobbish about this OP I think I get what you mean. You got through tough times by picturing a kind of delayed carefree fun teenage time with the two of you doing things you couldn’t do when you were that age. Getting drunk/going on hol/dating/maybe house sharing or whatever. But she’s now settling down and you’re realising that you’ll never do that stuff with her.

Well two things - one is that if she’s your first person to get married it’s hard to believe but married people do actually stay young, sometimes it feels like such a grown up thing that they’ll suddenly overnight become too sensible. Book one of those weekends away in now before she has any kids. And the second is that you can still do those fun single people things, just with your own friends and report it all back to her. You’ll be ok. At the wedding try to concentrate on the joyful facts eg that your mum is there to celebrate with you both.

How long has your sister been with her fiancé? Assuming it’s at least a couple of years if they are now getting married. Surely these changes happened when she first got in a relationship with him, her getting married isn’t really going to be any different to the whole time she’s been with him, surely?
Im sure you’ll still be able to go on girls weekends away with her and have fun with her, she’s not moving to the other side of the world. And if you think her getting married is bad, just wait until she has a baby!

I echo pp about seeking therapy so you have space and support to come to terms with your Mum's many years of illness and possible death, and the effect it has on you. You'll then be far less likely to 'cry ugly tears' at a public event in someone else's honour.

You also seem to have very fixed ideas about experiences that you feel you were entitled to, that you and your sister missed out on. That'd be worth exploring too, as very little in life is ours by right, and holding on to some imagined life lost will keep you stuck.

Is it possible they might be better?

It sounds like you’ve (both) been through a tremendous amount. I absolutely don’t blame you for being sad.
That said, try to persevere, and give your sister a wonderful day. You two are obviously close, so I can imagine a lot of feelings will come up for you.
However, there are some great breathing techniques that help with keeping calm. As someone who cries easily, I have found some techniques that stop me from sobbing uncontrollably, at moments I would rather not.
Deep breathe, concentrate on breathing through your nose, and hold your gaze up. Look on YouTube for guided breathing techniques.

You can choose to see it as mourning an experience that's lost (apparently), and worry about crying ugly tears.

Or you can choose not to.

It's up to you. You speak with such finality, as though something has been lost forever. It hasn't, or at least not in a way that every person on the planet doesn't experience. We can none of us go back to our youth, and many many many of us regret or mourn serious things that have happened to us - and none of us can go back in time.

Que sera, sera. You can dwell on it, get a bit tipsy and ugly cry and be morose. Or you can choose not to - you can choose to look forward.

I think you can and must control yourself here OP.

My husband’s sister cried (sobbed, bellowed, etc, I can hardly overstate how extreme it was) like she was at his funeral, and she likes me. It was so so so embarrassing. Please try not to. 😬

Why would you be crying sad tears

it’s a wedding. Be happy for her. She has found love

sounds like you need to broaden your horizons a bit

With respect, because your sister chose a different path.
You need to accept that she wanted different things.

I would have thought that crying at a sisters wedding is fine. What do you mean by ugly tears?!

I think you may be over romanticising about your early 20s and plans. It's impossible to know what life would have been like without caring for your mum, and naturally you're probably imagining a better life than you would have had.

I see where you are coming from. Try to get it all out before the wedding, and enjoy the day as a celebration for your sister and her husband.

I can control myself enough to not ugly-cry when it's not appropriate, especially when it would make me look like a dick and mar someone's special day.

I wouldn’t think much of it if I saw a bridesmaid but I would gently recommend counselling might help you make your peace with how things have turned out.

It’s completely normal to find comfort in imagining better times when things are hard but reality can never match that and things can’t often be put right. I was a young carer, my dad died at 12 and I left home at 16 so I missed out on travel and feeling carefree too and it felt so unfair. Therapy really helped me to accept how the chips fell and feel less sad about it all.

EDIT: posted prematurely!

With respect, you are making this all about you. How you will feel, how you will react, how sad you are (about something unfortunate that happens to lots of people) etc etc etc. Which tells me that those who get the impression that you would be attention seeking by 'sobbing' would be right.

Bloody hell, you’ve had some horrible replies! Ita not like you’re going to be wailing!

I cried watching Harry and Meghans wedding on TV fuck knows why now and I get emotional thinking about DD getting married, and she doesn’t even have a fiancé yet!!!!

Try to cry it out as much as you can before hand, give it lots of thought and process it all as much as you can. Then on the day the adrenalin of the whole thing will probably get you through. Oh, and take a nice hanky.

She’s getting married she isn’t dying! You can still do those things?!

Could you try to reframe your thinking? How lovely that after the battle you have all been through, your mum will be there at your sisters wedding. Your sister is building her family and perhaps your new BIL can be support for your mum to allow you and your sister to take a break and enjoy a carefree weekend knowing that there is someone there for your mum.

Yes your sisters life may change, eventually as children may come along but this won’t happen straight away.

Right, I unalived the poor OP's mum but doing the same to her sister is next level! 😆

My mum been through cancer and at my wedding my mum had active breast cancer and was in the middle of radiotherapy and my gran had terminal breast cancer and there was another medical condition with a close family member. No one cried...it was a celebration. A celebration that everyone was together and that's all that mattered. I honestly didn't care about my wedding (I didn't wear makeup, didn't get my hair done or anything like that. I was marrying a man who cared about me and still 20 plus years later cares for me)...the actual day doesn't matter it was amazing that my mum, gran and the other family member was there....cry before if you need you but celebrate with your sister...we never did the stuff you mentioned but we done a few things in the last 20 plus years after I got married. Celebrate and support your sister... Do the things you want too after she gets married. Enjoy the day. It go so quick x

You can still have weekend breaks when you’re married. My sister and I fulfilled a childhood dream doing the Sound of Music tour in Austria together when we were both married and I was pregnant. It doesn’t replace being young free and single, but it doesn’t mean that everything you’d planned can’t happen either.

I feel like I’m getting a really hard time.

You are talking about crying 'ugly tears' at your sister's funeral so are clearly planning this will be the case and wondering what other people will think. I expect they will think you are really attention seeking on your sister's special day or that you are pissed.

I totally understand you might feel sad/sentimental/grieving for what might have been but wasn't. Cry your ugly tears in your own bedroom at some other time when nobody is there though. Don't decide to do it then.