You can't get pregnant by rubbing something on your vagina unless there's sperm on it.
I highly recommend you do not wear your brothers underwear unless they're clean and he has not worn them.
You really don't need to worry about your period until you're around 16. If you haven't had it by then it would be a good idea to see your Dr.
Ummmmmm definitely not. You can not get pregnant from anything unless a penis goes inside of you and the male cums inside of you. Yes there isn’t chances of you getting pregnant from sperm touching the outside off your vigina because Some could end up in you’re vagina but those chances? Are very unlikely and very low. In other words, you’re fine wearing his boxers just make sure they aren’t wet but even I he cummed in his pants or sperm was on them and you wore them wet like that? It’s a very low chance of pregnancy especially if you haven’t started your period yet. I’d see a doctor but as far as the pregnancy thing goes don’t sweat It you’re fine but also... don’t wear your brother underwear anyways, it’s not somebing you should wear of his even for dressing up. I would just go to buy yourself a cheep pair of your own.