I don’t think anybody ever _needs_ to gain weight. It’s only an indirect indicator of things that matter (strength, endurance), and even those are only indicators what really matters: health.
And yes, I think that applies to anorexics, too, although there, of course, the only way to gain strength or endurance and thus get healthier is to eat more and gain weight.
I also am aware that some people train in order to look strong rather than be strong, but I think that should be discouraged. I don’t know you, but given what you say about your brother, it probably is genetics, so accept you won’t ever look as strong as some others.
Certainly, if you’re as strong as somebody who weighs more, you’ll likely beat them in exercises where you need to move your body (swimming, running, cycling, mountain climbing, cross country skiing, rowing, etc)