Happy birthday. I hope all your wishes, dreams, ambitions, and aspirations come true!
I'm sorry to hear that favouritism is the worst thing. I cannot directly relate to it because I'm an only child. Still, I've had situations where I listened to my friends about situations that they deal with because of the favouritism in their households, so I can only imagine how horrible it must be. I believe that it's best if you remove any expectations you have for your parents or sister to change and realise what they're doing is wrong, because, after all these years, they haven't changed. You don't deserve to put yourself in a position where you have to deal with their bullsh*t and ruin your mental health because of it.
Trust me, moving on and trying to avoid hoping for affection from them will bring you a lot of peace since you'll no longer have to hold any contempt in your heart because of their ignorance and lack of common sense to remember that they have (your parents) have two daughters and not one.
I hope that you'll be able to reply to this thread one day feeling much better and happier. You've done well, take a rest and enjoy your birthday. <3