Posh... tough talking here, but meant with love. Promise.
Why the F do you care what she thinks of you? You are doing the best you can. If she doesn't approve of you, so FFing what?
If her brother fancied you, and wanted a relationship with you, and her bitchiness put him off, then he is not for you. If he sat there while you were humiliated at your own bloody birthday party, then you need to get rid of the pair of them.
Would you WANT her for a SIL? I bloody wouldn't. SHE is not good enough for YOU!
FWIW, I wouldn't bloody give her the time of day if she'd have said half of what she has said and done to you.
I'm betting that if her life was as wonderful as she is projecting it to be she wouldn't be such a petty and nasty bitch. I'm willing to bet all is not happy in her home.
She's being nice(r), cos she has seen you off and you are no longer in the potential picture. I'm willing to bet that she is jealous of you for some reason.
You may have a lot going on, but FGS, ditch this Victim Mentality and hold your head up high. Sod the lot of em. Brass em out!
Stop feeling sorry for yourself, you are a good person with a terrific heart. You are getting far too wound up by very nasty people, that do not deserve your attention.
Take your DC to the events/activities, Nod and say hi, but that is it. Cut contact to minimum, phase them out.
When you start seeing in yourself what the rest of us see, a great mum, with a huge heart, sensitive and trying her bloody best, then you will attract friends that deserve you.
If you keep going around with this victim hat on, all the shitty abusive bullies will smell blood in the water and come to feed. On you.