Another man, because he's too dumb to use birth control, has become a soon-to-be Walking Wallet. She obviously wanted to get her hooks into him to be "guaranteed" that he will marry her. I hope he takes off. For whatever reason, single chicks in their early 20's seem to be ultra-delusional about the aspect of pregnancy.
As for your step brother, I hope he can be strong and grow himself a big set of balls and put his foot down on the matter of a sprog. Your SIL obviously wants a kid to guarantee that meal ticket if they should ever divorce. She can have a constant hand in his wallet for the next 18 years.
These men just don't get it. Once they are divorced, few women will want them if they have kids. It's baggage. You've got the brats to deal with PLUS the ex-wife/baby mama and all their drama to deal with as well. The constant monetary requests, problems in school, who the hell needs that shit? Then for the single moos, well, they are all out looking for a baby daddy/wallet to come and rescue them.
Aside from having kids at all, having kids out of wedlock has got to be the most absurd, fucked up thing there is to do with one's self. The constant bitterness and fighting of the child support checks, visitation, fighting with kid's pahrunt, being jealous over the other partner's new girl/boyfriend. For the next 18 years, like it or not, that other person is going to have a hand in your wallet and be part of your life because you were too stupid to use birth control or you had some delusional idea of what having kids does to your life.