Is Abby outdoor potty trained or pad trained, and is the boy trained? I agree with Chachi that if he is neutered, marking probably won't be an issue.
Max has an "only child" personality, but he accepted and enjoys the company of a brother. The key is to put Abby first for everything. She can still be queen.
If possible, I would introduce the dogs outdoors on neutral territory. Let them work things out between themselves. Don't interfere unless you see something dangerous. We had jealousy issues, but these were worked out fairly easily. Teddy was 8 months old when we brought him home and slept in an xpen next to the bed for a couple months until Max approved having Teddy sleep in the bed.
Best wishes in your decision.