Diff in experience I guess
As to willowleaf's post, I had already suggested that at least the OPer get some seat time in before being the person who got his sister and mother on the water. I just felt that the general idea of time on the water before boat purchase was within the range of what I had said.
My personal experience with newer paddlers who start out talking about staying upright as an important factor before they even put their seat into a boat is that they are often not just wet if they capsize. They are wet and panicked. It's not fun to look into the eyes of someone swimming and realize that there are no brain cells operating to work with on getting them out of the water, or every physical act that you request is met with it hurts, or I am cramping...
I've had this experience with acquaintances that frankly have no good reason to be so hesitant. There Adding age, physical issues and a family relationship is not helpful. Once you get them on land they have organized all of that into being good and mad at whoever suggested that this was an easy thing.
It only takes one unexpected flip. When it comes to family, my personal preference would be to take the edge off of that risk.
My husband and I have taken couples out to introduce them to kayaking, on safe local ponds so that they can get the feel of the boats (we show up with at least four on the car). In the majority of cases, we have advised that they get lessons from a local outfitter. It is good for our local economy and all anyway, but without that we'd have the same advice. There are usually such striking differences in fundamental comfort in the boats that we don't think anyone other than a professional should be in the middle, to avoid things getting personal.
Later update - I haven't commented on the intended environment, but willowleaf's descriptions of the river mentioned aren't encouraging. So far I have seem mention of boat wakes, commercial boat traffic and stretches of steep embankments that make it impossible to do a shore landing to solve a problem. If my mother did want to go out on that, I would hire a good guide - or two or three - rather than be in charge. I am just not that brave.
If it were a friends or buddies, what the heck. I'd say go out and stay near shore, and the worst that happens is an informative swim.