@JWChibi
You'd be surprised. Remember that she's still a child so it'd be irresponsible to have her ride a horse that is too big, but a small horse cannot carry an infinite amount.
Shetland ponies, for example, are highly recommended for new riders, but they can only carry around 60kg.
To add to that, this world doesn't have a lot that we have in our world, so it could very well be that horse breeding isn't as advanced as well. We've done a lot of breeding to get horses to where they are now. Back in the time of the Egyptians, horses couldn't carry people at all. In fact, that remained true for Europe for a very long time, due to a lack of controlled breeding.
So it is perfectly reasonable that she wouldn't start riding until she's around 60kg and complain about riding with her brother, taking the total weight way over the ~110kg a normal horse can carry without being distressed.