My (17f) friend's (17m, let's call him Oscar) mom have invited me to her birthday party. I have seen this woman two times in my entire life and we have hardly spoken, but I know that Oscar have talked to her about me a lot (he told me so). There will also be all his family and family's friends, and two other friends of mine, but one is Oscar former girlfriend (they're still very good friends) and the other is his best friend, so they both know quite wall mom and family. Should I go o politely refuse?
You should go.
His mom doesn't know you either but she has invited you and the other two of Oscar's friends as she knows that the three of you are important to Oscar. Go and meet his family, it's just one afternoon/evening. If you don't enjoy it leave early and say you've something to do the next day.