Robin,
Families can be really difficult. I don't think you're being petty at all; it doesn't seem like to o much to ask that you get to create a gift you thought of the way you want to. If I was your sister I would really appreciate that you were willing to go to the effort. I can really understand how you feel. So scrap away, use all your creativity and produce something beautiful. Might look better without pictures of your sister.