How do you deal with everyone else's stress causing you stress?

[Rockin'theBunnyFarm]

My two best friends are both going through some major crises, and of course coming to me for support. I don't mind it, in fact I'm happy to try and help them through it. But how do I know when their problems are starting to cause me too much stress for me?
On top of it, my mother has a serious mental illness and my father is going through chemotherapy and it's starting to get to him.