also, if u get the slightest notion that he may be developing feelings of that sort... then cut the communication down a bit. Although, i still think and agree with diamond, that a guy can admire you and like you for other reasons... not just sexual ones
But I feel that if there is a slight notion that he may have developed feelings for her, then is already too late. I mean, effectively, that is the end of a normal BIL relationship, because even if you cut down communication, there will always remain an air of uneasiness between them. There really is no turning back in such a relationship.
Best to keep a fair distance right from the start, be it a BIL, cousin, or some other male.