I like the first neck one best;

here is another, around the wrist, perhaps, with two flowers if there are only two siblings,or leave space for other family members, name on the handle of the sppon ?: http://caoimhe-aisling.deviantart.com/art/Celtic-Sister-Roses-79229605

Here are wings: http://msrain.deviantart.com/art/Celtic-Wings-tattoo-design-98928412

Here is something that could be modified; I liked the wrist design, the drop down circle could be replaced w/ a knot; or have two drop downs, one for each sister:http://wingsfire.deviantart.com/art/Tattoo-Design-02-17682626

This a knot; it would be cool if had 2 knots intertwined: http://msrain.deviantart.com/art/Celtic-Maze-10481824

Sometimes, I like to just go for simplicity. What is you had two female signs intertwined 3D style? http://depositphotos.com/8983174/stock-photo-male-and-female-signs.html with the words your name + hers, sisters forever entwined, through our family line