Committed Ceremony

I have been invited to a wedding in New Jersey. My adopted sister’s child father is marrying my cousin. He used to date my sister back when I was in middle school they share one child together. The wedding is in a few weeks I was told that only a few people would attend due to the location and guest limit. My sister is planning to attend, but I’m concerned it might not go as smoothly, as the groom contacted my sister a few months ago expressing romantic interest. My sister wasn’t officially invited but has informed him that she will attending since their children will be there. FYI the bride is the same woman who said she had hickies on her inner thigh from sitting on the toilet for too long.