Stripes all the way across are doable and will not look like 2 twins if you sew your pieces together on the bias. That way your seam allowance will be less noticeable. It will be more time consuming than just sewing 2 panels together but if that is the look you want (continous strips) then go for it. Post pics too. We all love to see pictures of everyone's work. :)
I just completed (last night) a raw edged appliqued heart quilt. It wasn't all that easy since I didn't have a pattern just a memory of one I saw at a quilt show. I washed it and dried it in the dryer. It looks nice--one of the few I ever completed and liked all the way to the end! So if raw edge is something you are thinking about, go for it. It is your choice since you are doing the work. But whatever you decide, your sister will love it because it came from you!