Should I let it slide and forgive or what?

Sorry but I do not agree with your mom. Respect is earned not owed. What if anything have you done to earn hers. If your judging her and talking smack behind her back, your wrong and so is the rest of the family that does the same. You should be respectful and polite. You only have to say a few words to her. Hi and congratulate her for graduating from college, when she leaves, say bye. I think you could manage to do that. It is the mature and responsible way to behave.Spent the rest of the time talking with others.