If both my parents have been under general anethesia with no problem before, what's my chances of having maligant hyperthermia?

Very small: Mh tends to run in families. However, it's possible to have ga without necessarily triggering the mh reponse, so there is always a very tiny chance that anyone could be susceptible to it, even without family history. Really unlikely, though. Your physician anesthesiologist is always watching for any signs of mh, and has meds available to treat it.