You are definitely not to blame in any way here.
In my book, no parents, whether related to us or not, are entitled to assume that we want to baby-sit either during the day or at night.
If we want to baby-sit we will offer, if not, we won't. We have had our children and brought them up. However fond we are of our grandchildren, only very few of us actually dared ask our children to have children!
People have children either because they wanted them, or because they were too lazy or incompetent to use contraception!
These days, it seems young parents assume their parents will happily look after grandchildren at the drop of a hat.
We made no such assumption, but paid a girl to baby-sit if we wanted to go out, and my parents would probably not have dared ask my grandmothers to baby-sit us!
If your relations by marriage insist on "talking to you" after hearing your step-son's complaints, explain quietly that there are two sides to this story. Yours is, that you at 65 are still working full-time and that you therefore neither wish to, nor are able to baby-sit. If they persist, tell them to mind their own business, or offer to look after the weans themselves!