My Boyfriend has a weird relationship with his sister?

Anonymous
I have been with my boyfriend for a year now (I am 21 and he is 20) and recently I have been getting really angry over his relationship with his sister.
They grew up together having a pretty bad childhood as their dad was abusive towards their mum, and so have been very close. (she is one year younger than him).
But there is closeness and there is inappropriateness.
A few months back his sister got her nipples peirced and she came down, pulled her top up, flashed her brother and asked his opinion on her piercings... and he had a good look at them and then started "spanking" her piercings with a plastic spatula and chasing her round the garden spanking her behind for getting them pierced?

He also "lays on her breasts" thats right. If we are watching a movie and he's sitting next to her he will put his head on her chest (38DS) and lay there because its "comfy".

I can't get over how weird this relationship is.
help please! how do i talk to my boyfriend about this?
My Boyfriend has a weird relationship with his sister?
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