"I've been following your blog for a couple of years now. As a woman I can't tell you how much I've learned about guys. Thanks for supplying such great information. I recently told my brother he should log on and read because during a talk he admitted he's strictly solo. He doesn't even date and insists he's not one of those incel guys. He just prefers jerking off. The problem is our parents bug him about getting married and having kids and then they whine to me that it's not happening. I'm getting fed up with listening to them and want to explain the situation but don't really know how. Any suggestions?"
My first and biggest suggestion is: Don't.
Not a word.
If your brother ever decides to tell them, he will. Though, in all reality, it's none of their business what he does with his cock. Beside that, you never know - someday he may change his mind about partner sex.
I once had a good friend say, "Jacking off is really the best because no one can do you like you do you." But that isn't necessarily 100% accurate, either. Humans are pretty good at learning. If you can teach someone how to, I don't know...say, type or play piano, you can certainly teach someone how to handle a hard-on.
What you may want to do, instead, is tell him to read the posts here on mutual masturbation. Sex therapists prescribe it for couples as a means of learning what their partner likes, what feels good to them, what leaves them satisfied. But the core lesson is learning how to communicate those things. If he is with an understanding partner, someone who is GGG (as Dan Savage says) Good, Giving, and Game), he just might move past solo masturbation.
But, beyond that, there are a whole lot of people who really do prefer jacking off. They've explored other activities and decided that, for them, going solo is the right fit. Who are we to argue?