It's not a stupid question at all, don't worry about that! No need to apologize. It's really tough to handle dogs with questionable pasts, and even moreso when you want to protect your baby. Has he shown any aggression toward people, or only to Rowan?
I'll try to only answer your question of how to keep yourself calm, instead of trying to talk about the behavior of a dog I know almost nothing about:
Definitely take precautions to make sure he's kept separate from other animals at all times.
Don't try to handle him if you feel unsafe or uncomfortable. Dogs can tell when people are tense or uncomfortable, and it makes them feel uneasy, too—and therefore more likely to react.
You might find it helpful to do some research; look into things like canine body language, behavior of fearful dogs, signs of fear aggression, how to act around shy & fearful dogs, etc. Understanding stuff like that can help you feel more comfortable around him if you know the best and safest way to act around him.
If you want some resources for these things, I could provide some links, or you can search the forum for all kinds of info.
If you start to feel confident after doing some research into canine behavior and learning how your parents are working with him, you can try interacting with him while your parents are there. But don't let yourself be alone with a dog you don't trust, for your own sake.
Try to take deep breaths and do what you can to calm yourself and reassure yourself that you're safe. And if you really think it's a bad situation and you don't feel like you or your other dog is safe when he's in the home, talk to your parents about finding him another foster home that will treat him well!
And if you aren't getting much help here, maybe try posting in the training & behavior forum. You might get more people with more insight into that kind of situation.
