Your next move is to do nothing. Next time however when she wants to leave to visit her family either go with her or she goes alone. Or don't buy the ticket at all.
I think it's excessive to be gone this long. We lost 3 parents last year and we were gone for a week each time. Not everyone has the luxury of being gone for months at a time nor would I do that to my husband or him to me.
And it is also the uncertainty, changing day to day based on her emotions. I would ask her why she told me something different the day prior and it turns into "I didn't say that", or "yeah but I cant leave my mom" etc etc
She also always said her mom ruined her first marriage, as she was married when she was 21 to 23 yo. I sort of see this happening again.. My dad would tell me "Go take care of your family". not "Please don't leave me"
I understand I should be there for her etc, but she knew for a good year this day was coming, and she was able to spend 2 months with him prior. I feel like I was supportive of her, always listening to her, researching cancer, working harder to afford her trips etc.
When she broke our last agreement again, and now will stay for Easter, I am just feeling very very low. Can I still have pride and continue to talk to her daily like nothing happened?
And yes, I understand she wants comfort and support, but how would she feel if I took her son away indefinitely, or for 2 months out of the country?