i think this is time for an intervention.
get your sister and tell you perants to sit down. tell them, your tired of hearing them argueing and yo either want them to go and get help, at this point i would search for a list of loacal marriage councilors, and say if they are so unhappy then they should get divorced bacuse they are making your life unbareable and of you cannot talk to them write a letter and have them read it in frront of you so you can make sure they are taking you seriously.
hope it works out x