Why would Derek and Meredith care if Lexie dated Sloan?

I'm still a little behind the series, I've just gotten to the episode where Meredith and Derek walk in on them kissing and it's all awkward.

Seriously, why do they care? Meredith has just very recently coming around to admitting Lexie was even her sister. I think it's kind of a stupid storyline because I don't understand why they would give a crap.


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