Because that's what happened yesterday at my DB and SIL's wedding, when my cousin refused to make eye contact with me, DH, DM, DCs and even the bride and groom.
She has always been a snob and a social climber and actually I can't stand her, but why turn up at the wedding if you hold the happy couple in such disdain that you can't acknowledge them? I found it stunningly rude.
There is recent history involving me and her parents, regarding children (ours) and spoilt snappy dogs (theirs), but we're all civil even though we no longer really see each other. My DM has a life limiting illness and although she isn't going to dwell on it I feel utterly offended on her behalf. I can't help thinking that when the inevitable time comes that we have to say goodbye to DM, she better hadn't come anywhere near us.