What was her response?

she says we're all going to sit down and have a "talk"

It is a same sex relationship right?

yes

Ok, if she says that it’s wrong to be in those kind of relationships during the talk, you should tell her you can’t help being attracted to people of the same sex, just like how she can’t help liking people of the opposite sex, and if she still doesn’t understand, ask her if she loves you, if she says yes, you should say she should not be offended by your sexuality, and if she says she should, then you should tell her that she doesn’t love you, she loves her image of you

she's fine with me liking girls, i'm just not allowed to date until i'm 18

Ok, then ask her why she’s not ok with it

she won't tell me
i think it's because she grew up in california and legally couldn't, and she wants to implement all of her old restrictions on me and my sister
for example, i'm not allowed to drink whole milk

That doesn’t make sense to me, you should tell her why you should be able to and that you are allowed to do it legally now. Don’t rush it, come up with good reasons why you should be able to

ok 👍 thanks

Good luck!