Girlfriend jealous with my sister?

First of all, even before we were bf/gf,

there was this one time when she saw me with my sis

in the mall (this happened early when were dating, I haven't introduce my sis to her), she was really mad and all but I explained it her.

In fact, even some of my friends (who don't know sis) mistook us as a couple (cause we don't really look alike, lol)

Anyway, my girlfriend still nags me every now and then about sis, like she ask me what we do, sometimes I told her like there are times that me and sis eat out (when none of us brought food for dinner or when she doesn't want to cook) and other things we do. I can say me and sis are close as siblings can be.

Is it me or she's asking way too much about my sis?
Girlfriend jealous with my sister?
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