Nemo gave you a really good answer. He may be unsure about how he feels.
More people than would admit it have done some "experimenting" with the same sex early on in life. Sex is a very complex issue.

I would agree that you shouldn't try to confront him about not wanting to meet your gay stepbrother. If your friend is having doubts about his sexuality, questioning him may make him defensive and extremely uncomfortable.

Maybe I'm wrong, but it seems like woman accept other women or men that are gay than straight guys do. (Guys don't always mind gay woman, but very often feel uncomfortable with gay men). We seem to be less tolerable as a gender about that than women. If a straight guy thinks another guy is making a pass at him or looking at him, say in a locker room, it can even end in violence.

Give your friend some space when it comes to that part of his life. If he decides he is gay, its up to him whether to tell you or not. He may not be answering your texts because he suspects you know about his struggle and is afraid of what you may ask or how you may feel about him. Understand that he is probably in emotional turmoil, and realize that he is the same person he always was. If your brother in law is open about being gay, he might be able to give you some insights as to what your friend is going through.

Sam2