He's attempting to keep all of his options open.
I used to watch a show called Nash Bridges starring Don Johnson and Cheech Marin. Don Johnson's character on the show had a habit of calling every guy he ran into "Bubba," and every female he ran into "sis" or "sister." It was just a verbal habit, like some people say "man" or "dude" a lot or they say "whatever."
Don't read too much into it. If your x is asking about your relationship status a lot, it means he would like to get back together with you. Otherwise, why would he care one way or another?
you should not have dated your brother?
tbh it probably means they are interested but not so much as to make a move, more to make sure you are available as a backup.