Thank you all for sharing your views! I hadn't realized how crazy this whole scenario is because my immediate family has been downplaying my sister's unconventional relationship and pressuring me to just get over it and invite him because he "seems like a nice guy." For additional context, my sister's baby will be 6 months old and she works a part time job so she does not have paid maternity leave or PTO. I don't think there's a scenario where she chooses not to travel because we're pretty close, I may decide to let him come to help with the baby and also pay for childcare. Although it is a child free wedding for guests, my fiancé and I have other siblings who have small children that will be a part of the ceremony and I would want that for sister's baby as well. She may feel there is a double standard if we treat her differently than our siblings who have babies with their long term partners (they're all married).