I wonder if they are also trying to reconnect, themselves, as you have said your DM went into a CH and DD went away to Uni (I think - I hope I haven't mixed you up with another poster). But, giving the benefit of the doubt, they may be trying to consider that your DM will not be cooking Christmas dinner this year, and that you may like to go to them this year, especially as SIL is home but not necessarily just because she is.
Now that is the giving the benefit of the doubt version.
I personally would be feeling the same as you and I don't know the history and back story. But there are family issues in our family too.
We now stay at our own house every second year, and only travel the alternate years. So we have a small dinner for us 3 (DH, DD and I) but still do turkey and the trimmings, and have family fun (we're so busy normally that a quieter afternoon suits us - but with presents, games etc still).
Perhaps, you could also offer to host either Christmas dinner (if it's not already been made clear that PILs WILL be cooking) as they may also like to get out from their house. Or else offer to have a family meal in your house another day that SIL is still around.
If you do have to consider the offer, you don't have to spend all day there from breakfast until almost bedtime. Go half an hour before lunch, stay another hour after it, but have other things arranged either in your DPs house or back in your own (maybe your DF needs Christmas tea if DM is not physically there anymore?, or you need to fit in with visiting hours?, or the hours DM will be released for the afternoon? or even that DD has other friends to meet so you are all heading together?....) that you need to leave for at a certain point in the day. Or maybe that you will have to cook for DF (and DM?) but will come over later when they go home for tea and stay a few hours in the evening?
Rant away, it's better to release such feelings in relative safety such as here (and possibly get good ideas and ammunition for dealing with your case from what others have done or what they see from the outside) than to simmer internally and it ending up coming out sarcastically or in a rage at some point and causing conflict - as you certainly don't seem to want any conflict in this. But don't feel guilty at not wanting to go, even if you know you will have to go at some point.