Before allowing him to have sex with you without a condom you should find out oral contraceptive pills for you to avoid pregnancy. It is up to you whether to fulfill his wish or not. In my opinion, you can have sex with him without a condom even on a regular basis too and can get the maximum pleasure but select your daily birth control pills so consult your gynecologist and ask her to prescribe you pills.
A few seconds penis inside your vagina without a condom is not required as his precum is capable enough to make you pregnant so if he wants to enter his penis inside your vagina without a condom then let him finish the session without condom and ask him to shoot his semen inside you but take proper birth control pills.
It really depends on how great he is with control. To be safe, try intercourse without condom only during your periods as the the slippery vagina is fun too.
I don't think it's right to do without condoms. You might try anal otherwise. Just get a good lube.
Try chocolate syrup on your brother's dick and it shall be better to give blowjobs, after all he should be getting his fun too.
Yes, you can have vaginal sex without condom but you have to ensure that he doesn't ejaculate inside your vagina.
There shouldn't be a single drop of semen sticking to his penis when he enters your vagina without condom else you might get pregnant. Alternatively, you can have anti pregnancy pills also to reduce the risk of pregnancy.