Celebrations among family members are often an important part of their social calendar, so a cancellation is not always greeted with calm. The host or hostess for the event might begin immediately calling around to try to find a guest to fill and empty seat, and they will ask everyone if they know of anyone willing to attend. They prefer a full table, and a new guest could be very welcome. They might ask if anyone has a friend from work available, and they are seldom choosy.
Seating arrangements are often traditional at family gatherings, and the unexpected guest will likely be seated where the missing family member should have been. If they are next to a single person, suspicion might be noted. While their hackles could be raised, they should relax because this person is not necessarily there for tempt them into a relationship. Their sole function is to fill a place left by a beloved relative unable to attend.
The person filling in for the missing family member might know a few people, but it could be a bit awkward when they first arrive. Setting them at ease is the duty of those hosting the event, and the person who asked them is expected to introduce them around. Sitting next to someone hostile could be embarrassing for the entire family, so attempting to at least make casual conversation is expected. The two of them could hit it off, but each of them should know it is not a goal the family has sought.
Meeting someone new at a formal family function is generally interesting, and it could lead to something more. Chances are it will be nothing more than a conversation with a guest the single person might never again meet, so they do not need to be anxious they are being matched up by loved ones.