I think I got an answer as to why the SIL is coming. It's not about my daughter's big day. It's about her, naturally, being the narcissist that she is. My BIL has made this into a road trip for my SIL where they are going to drive down to our house and visit her relatives in our city, then continue driving to visit her other relatives. My BIL is not even going to go to the actual graduation, just attend the party afterwards at our house with SIL. So we are just going to be a quick footnote in what is otherwise a trip planned mainly to see SIL's family. Oh well. Am I surprised? NO. Am I disappointed? NO. Seeing as my daughter and the rest of us are uncomfortable seeing them in the first place, it's better their entrance and exit be quick anyway. My BIL has acted annoyed at having to come to our house. I'm sure it's because of nasty SIL who probably feels forced to make the appearance and doesn't want to. So my BIL's angst spills over onto us. Boy oh Boy!!! There is a special place in Hell for toxic people like my SIL.