Here is what the contract says, regarding "family": "For purpose of this subsection and subsection iv, a character is “being used by MCA” if (x) it or another character of the same “family” (e.g., any member of THE FANTASTIC FOUR, THE AVENGERS or villains associated with a hero being used) is more than an incidental element of an attraction, is presented as a costumed character, or is more than an incidental element of the theming of a retail store or food facility; and, (y) in addition, if such character or another character from the same “family” is an element in any MCA marketing during the previous year. Any character who is only used as a costume character will not be considered to be “being used by MCA” unless it appears as more than an incidental element in MCA’s marketing."
So, you're saying that the Guardians fight Thanos (I'm not familiar with it enough to know myself, to be honest with you). I don't thank that would take away from Guardians being defined as their own family, as other than that, they wouldn't be linked to The Avengers. However, it could put into debate as to what family Thanos is in. Given everything I've now read about the Thanos, it would be hard to argue that he is not a part of The Avengers family. I think the line, given from description in the contact, would be drawn at direct core members. So all of the labels in the Avengers, plus the villains. If one of them crossed into another universe or "family", they would still be considered apart of their parent family and would be like a "guest" in the other.
So, given my experience with law, I don't think there would really be much to argue in terms of whether Guardians is in The Avengers family, given the language of the contract. At best, Universal could attempt it to simply delay any of Disney's plans. A good lawyer, which Disney has plenty of, would be able win this one, imo of course.