Have you actually spoken to MIL about her thoughts/plans to move closer to you? By you, I mean YOU not your DH. If not, could you start a conversation with her, and tell her your concerns for her well being, and that you and your DH may decide to move to a different area in a few years time, when the children are older. Hearing that alone, might be enough to make her think twice about the location she's thinking of moving to, as obviously she's not going to want to go to the expense, and emotional uproar of moving, only to find that you up sticks and move away. Ask her her reasons for wanting to move, if she says it's so she can see more of your children, point out that they too will be grown up within a few short years, as her GC elsewhere have done, and so while she might have a year or two of seeing more of them than she does now, it won't last. As a woman, if you get on reasonably well with her under normal circumstances, she may be more chatty about the idea than she is with her son, plus, often things are lost in translation, ie, your DH might tell you what he thinks he's heard his DM say, but in actual fact, she might be saying something totally different. I've found with a lot of men in particular, that if you ask them about a conversation they've had, they can't remember most of what's been said, as they don't tend to listen in the same way that women do, and also don't tend to pick up on the nuances of conversation, like a woman does either. I may be totally wrong, and your DH might be perfect at explaining things, and understanding what's been said, but the book 'Men Are From Mars, Women Are From Venus', didn't come from nowhere.
I would try and make time for this conversation sooner rather than later too, as while I realise your mind will be pretty much focused on your own Mum at this time, which is as it should be, if you leave the conversation with MIL too long, you may find that in her mind a move is done and dusted, and it will be a lot harder to dissuade her from her plans.