I wasn't aware that I had said that my Mum didn't do any donkey work Owllady...she did, but it was as and when she could manage leave from her job, and pay for the flights to see my db and his family, or afford the petrol to come and see us.
Not every one lives close enough to see each other regularly; parents may still be working, and have other commitments, like other dc still at home, or have caring responsibilities for their own parents.
You are only going by what the OP has said. For all you know the OPs Mum could have a raft of responsibilities that haven't been mentioned on here. Her holiday could have been bought and paid for before the Op got pregnant. Is the Mum supposed to put her life on hold, just in case her daughter conceived? She may have been travelling with someone else who would also have had to forego their trip if she pulled out, or pay more if travelling as a single. Would the OP have been prepared to cover that cost?
My Mum got little help from either her mil or own mother with my db and I, not because they were avoiding donkey work, but because we were a Forces family, and moved every couple of years to different parts of the UK. My gps lived in East Anglia. I had a close and loving relationship with both sets of gps, and when my Dad died in 2001, my Mum, db and I took over the responsibility of Nana (which was v selfless on my Mum's part as she and my Dad had been divorced for quite some time by then).
I was grateful for any help from my Mum and pils when ds was small, but it wasn't my right to have that help; it was for them to offer it as and when they could fit it in with their lives and commitments.