I totally agree with Johanna. No reason for an "excuse" - fake or not. Just a "I apologize that I will be unable to attend" and a gift sent in your stead. Or wait, and give the gift in person when it's just the two of you.

Don't feel bad - I feel the exact same way about my soon-to-be sister-in-law's family. They are trash and her sister hates me because I offered to help out with the wedding (I'm not just "some friend" of Shannon's, I'm the sister of the GROOM!!!) AND because I am pregnant and she cannot have children. The bridal shower last month was TONS of fun.