For and foremost, in a situation were you can't be 100% sure your partner's family will respect you, get the papers in order so they can't exclude you from, for example, decisions you hope will never be necessary but unfortunately might be: if they are hospitalized, decisions about a funeral, decisions about what to do with your partners things after their death etc. If you don't do anything, you have no legal standing whatsoever, and that can be a tragically bad idea (as if such a situation weren't tragic enough).
So get something in writing. Even if it's not marriage papers. #legal_yuri_advice