I agree with the comments on the 21yr old. But maybe it's not so much him expecting it, as OP just doing it.
Certainly both my 14yo and 11yo would manage fine getting dinner, and I suspect my 7yo would be as long as he wasn't using too sharp knives. They'd probably quite enjoy it. You could send the dog on the narrow boat too, as they're ideal for them, as security as well as they can walk along with the boat.
But, having been on several canalboat holidays I can't help wondering what on earth your bil was thinking.
Assuming they're planning on moving it further than the nearest pub and back that is.
Firstly you need to be able to get on and off occasionally quickly. That's not that easy, and we seemed to usually have to jump a crop of nettles when we landed-usually into mud! Any water on it, and the sides can be very slippery.
The beds are small, and a choice of very low to the ground or top bunk. Not the easiest thing to get in or out of.
If she's any balance issues she's going to struggle moving about-the toilet isn't the easiest place to balance either. 
And then we have the work load.
Assuming she isn't driving, which I might be wrong on, it's going to be one person drving, one person operating locks. Now locks aren't dreadful most of the time, but actually it's pretty boring jumping out, opening the lock gate, open gate, wait for boat to go in, reverse process. With two people it's okay, you can chat and help each other. One person it really gets to you. There may well be areas where you are just walking from one lock to the next.
Then what's your mil going to do? She's going to spend most of the moving time sitting on her own. Because you don't generally want to sit up with the driver as the engine is too noisy for chatting nicely. I liked sitting on top of the roof best, but I doubt your mil will be up for that.
The boats can also, even with heating, be very cold. Dh remembers one trip at half term where it was so wet that waves were breaking on top of the boat as they drove along.
It's not really a nice holiday for 3 people, even 3 able and willing adults, in beautiful sunshine. For 6 people all of whom can operate locks and/or drive it is nice.