I got the Liquid Logic VersaBoard 3 weeks ago. I have been on quiet lakes and some moving water. While standing, I cannot keep up with people in touring kayaks, but I can keep up with my friends who have 10' play kayaks. In faster water with riffles, I have to sit down to paddle because my balance needs work. I want a backrest for the seat.
My Golden Retriever is riding in the bow of the SUP, but I think the deck is too slippery for him. I'm going to put a non-skid bath rug on the bow to see if it makes him happier. Photos soon.
Kiragirl, I'm not a surfer.
If your sister can stand up and paddle, then it can be used as a SUP, whether your BIL likes it or not, right?
The key to anything that floats is displacement, which equates to volume. The more water is displaced, the more stable it becomes. It can be wider or longer. You need a longer board in rougher waters, too. The rocker (curve from nose to tail) plays a role in stability as well. Low Rocker is better for a SUP.
I liked a SUP that was 11'6" and 32" wide. A Rubicon SUP that was 12' and 30" wide was waaay too unstable for me. It just depends on the rails, the rocker, experience, and lots of other factors.
You are also more stable while moving than just standing still. Compare it to a bicycle. While some can balance standing still for a long time on two wheels, it is much easier to balance on a bike when you are moving.
Get her a SUP paddle and see if it works. If not, return the paddle. As long as it is in resale condition, I don't think it would be a problem. OR have her try a boat oar that is about 10" longer than she is tall. It won't be as efficient, but it can give her the feel of her board as SUP. If she likes the SUP sport, I know what she can ask Santa for.