You are right. I don't know for certain that the families are defined, the contract doesn't specify exactly what is in those character details. But because it isn't stated specifically we know that you can't say that GoTG automatically will become Avengers family when they appear in Infinity War. The bottom line is that we don't know what is in those unattached schedules.
If your thinking was correct, then as soon as they appeared in an Avengers comic or cartoon, then they wouldn't be able to do Meet and Greets with GoTG or Dr Strange at DHS. The MCU could not be more heavily weighted than any other media, unless that is written in an addendum that we can't see.
My point is you can't say things with any absolute certainty. We simply don't know what is specified in the unattached schedules.