Thanks all for posting, I do appreciate hearing everyones (polarised!) thoughts.
I am going to speak to DSs teacher (who is also the Deputy Head) on Wednesday, when I'm next doing pick up and ask her about typical workload on a Friday afternoon. Explain my concerns and ask in this instance, if it's possible to get extra work to compensate etc.
DH isn't being brotherzilla. He just doesn't like confrontation, especially with his family (which frustrates me to the core!). But then I'm also a bit of the alpha-female in our relationship, seriously, my own family joke he is 'under the thumb' a bit. I'm as stubborn as a mule, he just gives in for an 'easy life', whether that be me or his family.
We all definitely want to go, as a family, for the day itself. That isn't under question. It's just I'd feel more comfortable getting back for DS to get back into school late morning on the Friday. We're just deciding the best way of doing that.
Thanks everyone for your thoughts. I'm surprised by how many have said they agree my SIL is being inconsiderate and even more shocked by how many say they wouldn't go at all (this isn't something I would contemplate doing).
I do think those who say "only those who care enough will come" is a bit strange. Of course I care for my SIL, but at what point does it become too much? If it was like a 2 week wedding abroad, £5,000 flights, hotel etc, is that then becoming unreasonable. Surely it's a sliding scale? It's not about how much do you love or care for someone. It's about looking forward to enjoying their day, without fear of family battles at being seen as unreasonable because it is inconvenient, and trying to get to it without sacrificing other stuff. I also think those who have arranged weekday weddings because their nearest and dearest find weekdays easier (or in school holidays) are very considerate.