Is my boyfriend attracted to his sister?

I've been dating my boyfriend for about 2 months. He's made it very clear that he loves my butt (it's pretty large) and is always talking about how nice it is, how much he loves grabbing it, etc.

However, about 4 times throughout our relationship he has compared his sister's butt to mine. He will send me snapchats from her story of butt selfies she takes and he'll be like "my sister has a big butt just like you". Recently, he told me that his sister was nominated to be voted for "best butt in the school" at her high school and he told me that he thinks she'll win because "She's pretty so she attracts a lot of guys".

Do you think he is just comparing my butt to his sister's as a way to bond with me? Or is this something i should be alarmed about? I'm feeling very conflicted so any opinion would be appreciated!
Is my boyfriend attracted to his sister?
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