I would like to make my sister and my niece (her daughter) a b-day cake. Their birthday is on the same day so I would like some ideas for something that would work for both of them. I'm still new to decorating so it can't be too elaborate just something simple but nice. Any ideas?

Well.....they are both girls, so they might like flowers. What about their favorite colors with some fondant cutout flowers?

That is relatively simple......

How about matching Mother and Daughter cakes...one regular size and a duplicate mini-cake? I like the flowers idea. You could do fondant daisies in really bright "girly" colors. If you do a search in the gallery for daisies there are lots of cute ideas.