SORRY I meant do you think its weird when a guy dates a girl then goes out with HER sister?

I clearly made a typo and I thought I wouldn't have to re-ask the question, but people kept saying **** about . Anyway, most dudes I know think its normal for a guy to date a girl then date her sister. Well I went out with this guy 3 years ago (yeah I know) for about 4 months and he was my first love. Now he's dating my sister. I know I shouldn't care too much because it happened so long ago but I can't help but remember all the times we had when he's at my house with my sister. And I feel kinda gross..like I can't even think about him cus that's my sisters boyfriend..ew >0< anyway, what do you guys think? Everyone around me seems to this its normal /: