Is it normal that i am attracted to my twin sister's tits

Is it normal that i'm attracted to my twin sister's tits. We are both 19 now and we are sharing an apartment with each other. Anyway so one week ago she had just got out of the shower and she had forgotten her bikini top (as she was going to the pool.)So I got a good long look at her huge tits. At first i thought she was teasing me but I knew she wasn't. So I helped her find it and I put it on her breasts zand held on for like ten seconds. Is this normal or not