Is it normal to want to see my brothers penis under his kilt (when he wears one he doesn't wear underwear) though I am a lesbian.?

Well, I would say most women are not drawn to the sexual organs of their gather, brothrs or first degree relatives. It doesn't mean your insane but you may need a bit of psychotherapy. Just because it's a boundary issue that is unusual. Was a boundary ever crossed physically within the family would be the first question. Good luck, your not majorly flawed, but think about the boundary issue.
Not unusal. It sounds like you have normal urges, when it comes to sexuality, despite your orientation. Certainly you should not act on your urges with your brother, but you might find that you are in fact, bisexual if you enjoy men too.