A real individualist vs collectivist thread..
The modern western individualist thinking would be that:
They chose to have the baby. They are independent adults, and should be capable of dealing with their decisions
You are your own person and should be able to do what you want with your own life, if that is helping - ok - if that is not helping - ok - neither should be wrong.
The more older/traditional collectivist mindset would be that:
Responsibility to those around you (family) comes before personal satisfaction/happiness. Of course you should give help in anyway you can. Equally you should be able to expect help in return form those around you. If we all pay our responsibilities to each other, we all benefit together.
Personally, I'm not sure which one is better... but what I am sure of is that everyone in the family needs to be on the same page. If you are willing to give up your happiness for them, you should be able to expect that in the future they do the same for you. And if you think that they won't, they will act as pure individuals when it suits them, but demand lots from you, then you have a problem. Only you know that.
If you got sick in a couple of years, and needed your son, would he drop his holiday and come to help you? That will be a telling answer, and guide your own decision.
Personally, I would expect my mother to come if she could. But equally I am 100% prepared to drop what I am doing and come to her, if it is possible as well.