What is the cup size of my breasts? 😥

My lower bust is 31 inches and my upper bust is 36.5 inches. What is my cup size? What bra size would be right for me? And what is sister bra (I can't remember if it's called sister bra or something) ? Here bra size is measured by inches. For example I can both use 34 and 36 (sometimes 36 is smaller than 34) Last time I went to buy some and the salesgirl recommended me to buy 32 ones, being confused I came back. So please someone help me I'm so confused 🙁