My bf's brother is gay.we don't know if his bf has stds but if he does and it's transmitted to his brother would my bf get i2 if using same toilet?

Absolutely no risk: HIV and STDs are only transmitted by sex or blood. Nobody in the same house of someone with HIV, syphilis, herpes, HPV or other STDs -- sharing bathrooms, kitchens, etc -- ever is infected unless they also are sex partners or share injection equipment with the infected person. Profile says you're male and have a bf. If so, there's your only HIV/STD risk. Be safe and get tested from time to time.