It could be a number of things but the main reason is because your brother is being well a big brother and taking the offensive approach cause he sees yall getting serious or another reason could be that youre bf is gettin more wrapped up in you then in bein friends with your brother, if I were you I would sit down with each of them in private and talk to them about the other one and ask why they have become so distant from each other and then try to work from there...good luck!!!