Moving house has been compared to divorcing in terms of the stress level.
How many times are you going to put yourselves and your DCs through that amount of stress to please your DM and step father.
What if you can't afford to rent, can you live permanently at FIL's? With all your belongings, without hindering his home life?
I know you think your DM is a good grandmother, but I don't believe it. I understand what you said about step father controlling DM, and maybe she needs help because of that, but losing large amounts of money you can't afford and having no home to live will not help her and it could negatively impact your DCs. That doesn't achieve helping DM either.
As for the potential money laundering aspect, a lot of money laundering enterprises actually appear to lose money. The people who are doing it don't care about those loses because the money they get back is clean. The financial authorities know that, that's why simply saying "but I lost money" won't satisfy them if they suspect wrong doing.
I'm really sorry you've been put in this position, but I agree that you are taking it far too lightly.
If I were in your position I would not only get legal advice but I would instruct a solicitor to write to them outlining the agreement you made with them and with a sensible proposal for resolving this. Like if they want the investment back they relinquish that which they invested in.
And if they cry "you set a solicitor on us" they tried to make you and the GCs homeless and out of pocket.