Maybe not being mean? Maybe he thought that at 81, it's a bit tough for her to do the whole thing - and if he had to choose between the day and evening, she might prefer to sit with quietly with family, and leave in her own time, at the evening reception? Rather than stand around for ages, possibly in the heat of a summer's day, chatting to people she doesn't know, be faced with a lot of alcohol and food she might not be able to eat, and sit on a hard chair while listening to speeches?
It might not involve all of those things of course (the chairs might be padded!), but the average wedding day is a bit of a marathon, even for the hale and hearty.
I'm sure there thousands of octogenarians for whom that would be a walk in the park, but for many it'd be really difficult. Obviously you think your mum is more than up to it OP, but your nephew might not have the same experience of her as you do.
Or, maybe the bride is only inviting her grandparents to the evening do, and it seemed fair and reasonable for both GPs to be at the same event.
Anyway, my advice would be to find out what's what, before jumping to the conclusion that he's an uncaring nephew. Check with him - or your sibling who's his parent - exactly what's involved in both the daytime and evening events.
You don't say what your mum actually wants. But if you can give her more detail about what each event involves, and she says she'd rather go to the daytime ceremony - then you could ask your nephew or sibling very politely if she could be included. But ultimately, as PPs have said, it is nephew and his wife's day.