I'm not really that creative either, but I have often taken quilts that I saw in magazines and adapted them to what I want. I have a friend who also quilts and we bounce ideas off each other. She's not real creative either and neither of us get upset if one of use gives a negative review. We also use graph paper and colored pencils to sometimes draw out the quilt and we both have design walls.
As far as your sisters idea, maybe you could tweek it just a little to make more of your idea. It is hard to come up with ideas, but sometimes it helps to Google quilt patterns and you would be surprised of all the ideas you can get from there. Good luck with your quilt and be sure to post pictures