My stepmom told me that she’s already pregnant. what should i expect in the coming months?

You are handling this great! And the fact that you have a great relationship with you stepmom makes the whole situation even better. You can pretty much expect her to be tired and to get a little hungrier. She may have some mood swings. After the baby gets here, try to be helpful, offering to feed the baby, change the diaper, watch the baby for an evening while her and your dad have a night out. (they'll need an evening a lone) Congrats to you big brother!