Rules:
you mentioned that your daughter and her future hubby are not Jdubs. They should sent invites to whomever they want at their wedding. If different ones want to boycott the wedding, I guess its their choice. I think it would be more of an issue if your daughter and future son- in-law were witnesses.
I had a similar situation when I got married. My SIL married out of the troof and some boycotted because she was "marked" . Some felt she should not have t been allowed to attend. I didn't care. They were the losers as they missed out on a great wedding.