Mamma66.
I'm so glad you have decided to go to the Party on Saturday and "behave with good grace and rise above it". Good for you! It's not easy, when you have felt hurt, but you've done really well to come to this decision. I think going to the party in this way will help you, now and in the future. You sound much more confident. Don't let anything pull you down. Michelle Obama said, when asked how she coped with insults and nasty comments, "Always go above them". She was saying what you said really, be gracious and hold your head up and don't let them pull you into any kind of nasty row or horrible atmosphere.
As you can see, many of us have met people, within the family and outside, who have been upsetting towards us for no reason. It just happens. I can even remember a Teacher who came to our school when I was in the sixth form and for no reason any of us could work out, she kept picking on me. I had been very popular with the Teacher she replaced and always had good comments on reports from all my Teachers. But she could have ruined my school life. I had to be really tough to keep up my morale and know I was not the nasty person. When someone picks on you the only way to survive is to look after yourself and not take them seriously, and never get into arguments with them. Remember who you are, what others think of you. There will be plenty of people at the party for you to be with so you can relax and enjoy yourself and keep away from her.
I do hope that in the future you are able to maintain those relationships you want to keep without any interference from this person. There are lots of good ideas here from these wise Grans on how to meet up. As for the SiL, try to minimise the effect she can have on you, remember, it is she who has problems not you, and avoid her as much as possible, and avoid arguments with her, even leave the room if she starts picking on you. Just smile and say, "Excuse me, I must just pop to the loo."
Wishing you much happiness. I hope that your brother's worries soon are overcome. Wishing you all a great time on Saturday and that you all enjoy the 21st Birthday Party! (Please let us know how it goes! If it is not a cheek to ask!)