To expect more...

[MildB]

It's my son's baptism soon, and before we organised it we asked all immediate family (ie grandparents) what dates they can/can't do. We found a date which suited everybody, but now, not long before the date, I'm being told that MIL will only be coming to the actual mass, and not the celebration afterward, as she's going to her boyfriend's grandchild's party - not her birthday which is several days later, but her party. She never mentioned this when we were organising which date suited them.

MIL has 4 grandchildren and she only sees 1 on a regular basis. She also sees her BF's children/grandchildren regularly. We are all close by, but she says she doesn't have time to see everyone (we see her perhaps once a month). She won't be able to give DC a cuddle or anything during the mass, but would have been able to have quality time with him afterwards.

AIBU to expect more from her?

Probably not. But baptisms and christenings are the most boring events known to man. Actually, second only to baby showers.

She’s coming to the ceremony and is missing the social bit afterwards but how likely are you to hang out with her anyway if you’ve got loads of other people attending?

It sounds to me that she's going to the most important bit.

Is MIL religious? Half of my family are and the other half aren't. The ones that aren't don't understand the importance and would probably do the same thing as your MIL.