My wife's sister all ways grabs, pinches or smacks my ass and I'm wondering if I should say something?

Anonymous
She is quite a bit younger than my wife and by nature is kind of a flirt. I'm not thinking she's got a thing for me but I am starting to feel uncomfortable. I'll give you an example, about two weeks ago she bought me a pair of pants for my birthday and I tried them on, when I came out of the bathroom she was waiting outside the door and walked behind me and grabbed my butt and said how nice it looks then kissed me on my cheek (which to me isn't a big deal. She always does.)
There are similar examples. What bothers me is she only does it when we're alone which has me wondering if there is more to it and also the fact I wouldn't even think of doing it to her. Not even in a playful way. I know she's young so should I just let go? Do any of you have a relationship like this with a siblings partner? What are your thoughts?
My wife's sister all ways grabs, pinches or smacks my ass and I'm wondering if I should say something?
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