Bra sizes are confusing ...

Hi

I am trying to buy bras for my self, but realized that I have no idea how confusing (and expensive😣) everything is?

Up until now I’ve been wearing 32a bras, but now they grew and I don’t know what size they are.

I also hear that you could be one size in a push up bra and a smaller size in a sports bra...or something like that (something about sister sizes).

Anyway can anyone explain how to figure out my bra size or what is a sister size? (Thank you in advance🙂).