There is a lot unsaid here and so it is a little hard to advise fairly.
Start by taking a deep breath. I'm sure you love your family and want to protect them. This is hard.
I don't think there is much you can say to change what is going on, but you can offer your support. What is your brothers situation? Is he working or in school? If so. If he is productive, would you be comfortable offering for him to stay with you temporarily. If this is not something you would be comfortable with then just lend your brother support emotionally. Tell him you believe him and let him vent to you. You cannot control what goes on in someone else's home. It probably will not be helpful to try to intervene either. Your father, and your brother, are old enough to make their own decisions. Your brother is old enough to find alternative living arrangements if he is unhappy there.
I'm trying to tell you that you can't really come in for the rescue.
You say "I also heard my step mom might be thinking about an apt." You leave a lot unsaid. Where did you hear it? Is it a secret? Again, you can't control anything, but are you comfortable supporting your Dad emotionally? Ask him if everything is alright between he and his wife. Let him know you want to support him...... not tell him what to do or put down his wife.
You can't fix it, but you can love your family.
Hope all turns out well.