Can your husband arrange something with your MIL? If she really meant what she said, she would agree to something if her son was to sort it with her? Sometimes being the DIL trying to organise everything can be awkward (I feel that too at times)
Is it possible that potentially she realises she agreed to too much with your BIL and his family, and actually now has limited capacity (mentally and socially) for more, and maybe feels she can/has to draw the boundary somewhere, even it is not perhaps the most equitable overall, as once she agrees to something more she will not ever be able to step away. She is already full of obligations and would love to see more of your kids but she also doesn't want to sacrifice the obligation-free time she has. Perhaps she sees you have your mum who can offer up help and feels you have the support you need. Possibly she is overwhelmed with what she has agreed to already, perhaps she is concerned she will never have time for herself to enjoy retirement because of a commitment she already made to your BIL which she feels she can't step away from. I would suggest it is potentially unreasonable to expect her to reorganise her non-obligation time.
Either way, it does come off cold on her part. However, perhaps DH husband can sort times for her to see the kids adhoq as it was to him she made the offer? Otherwise maybe just lower expectations and view any time she does spend with them as a bonus.
It is always so surprising when you have kids that the people we expect to help/be engaged/supportive are often not there for us how we would have imagined, and the help/support/interest can often come from the most unlikely of people!
Sorry you are feeling this way. Would recommend trying to reframe, remove expectations and move forwards xx