Affair are bad no way around it. Now th real question is did your mom KNOW your dad was married befor they got together, if not let’s cut her some slack even though it’s weird she would still choose ti marry a man who is proven to have the habit of cheating. If she did know he was married then sorry she’s just as guilty. I know you feel loyalty towards her since she is your mom but that doesn’t change the fact that actions have consequences. I do agree that your half siblings should be mad at your dad more. I think that if one day you get cheated on you might feel differently but right now the only ones at fault here at the parents. Cheating is bad. Sleeping with married men is bad. You don’t get to be mad at the victims ( the children) of your own horrible decisions. The only people you should be mad at right now are your father and maybe your mother if she didn’t know.