okay, this is a difficult one...
hope my explanation will make sense, so here goes...
first of all, you have a relationship, it is normal to look at other guys and fantasize, means you are normal. but if you really love your boyfriend, you should stick with him, try to forget the bro and see him as a bro and not a suitable husband.
second, if you are not fully satisfied with your current boyfriend, then consider if your relationship is worth the time and effort. i can imagine that it will be a great shock for your current boy if you are dating (will be) his brother.
be open in your relationship, but not to open, make up where your boundaries are, sometimes it is just better to keep quiet.
third, you are not sure of the feelings of his brother, so try to talk to him about it, is he seeing you as the girlfriend of his brother, or as a stunning beauty for himself
. talk to him when you are alone somewhere... deal with the situation, otherwise you will spend a lot of time and worry about it.
if you choose for the bro, remember that it is fully your choice... the situation can be awkward, but so is life.
think about your relationship with both very hard... and choose one, stick with that decision and know their will be tears and heartache. this isn't simple
good luck and all the best