God, some people really are taking this a bit far and being very unreasonable themselves!
The OP, who is pregnant, should not have more children because she'd like a lie in for 1 hour on one occasion? Really?
OP has already said her MIL was awake, so not like they are waking her up. OP has also said they do cooking/dishes and shopping (and presumably pay for it) while visiting, so it's not like her MIL is having lots of extra work. MIL is not "very old" and seems to be in reasonable health at present according to OP. In fact, OP and her DH helped her after a recent hernia op, so it's not even as though it's all one way traffic.
It's true that GPs are not obliged to help their children in any way once they are adults, nor are they obliged to help with their grandchildren in any way whatsoever. You'd hope they'd want to, especially if they only saw their GC very occasionally, but they aren't obliged to.
Reading between the lines, I get the impression that the OPs MIL doesn't show interest in other ways too- it's not just not wanting to get up early (OP says she can't ask her to take DC to the park or cinema, for instance).
My mum looked after my DD (including getting up early) when I was really tired and stressed while studying for royal college exams and working full-time. She didn't have to do it, but recognising how tired I was and because she's my mum and she cared, she did it. She has my eternal thanks for that. She actually offered when she came to stay, but if she hadn't I would have asked (begged, actually). I would have been disappointed if she'd said no (I wouldn't have said or done anything about it). I guess I'm very entitled.
OP- YANBU to wish your MIL showed more interest in her GC and disappointed she wasn't willing to help out when asked. Yes, it's her right to decide what she will and will not do, but you are allowed to be disappointed too.....
If this is a pattern, rather than a one-off (i.e. she doesn't show much interest in her GC, your DH or yourselves, nor does she choose to help and support you very often), then I wouldn't be running to her aid next time (unless it's an emergency). Care and support is a two-way street.