@PS92 I get what you're saying, but really it’s none of their business what happens to monies given to their DS or his siblings.
For instance, had DH been unable to conceive naturally and they’d gone down the IVF route, perhaps used donor sperm - would ILs be justified in getting antsy about where ‘their’ inheritance goes and excluded non-biological but legally his children from their will? I hope not but I wouldn’t be surprised.
If DH’s siblings get divorced at any point, lose half their assets - or more if court awards potential ex wives a larger share to purchase a home for their children, or even if they are offered their homes in a settlement - would they have any standing in court proceedings to have their gift deducted? Nope.
I think this has been triggered by OP/DH saying that they are looking to move and reveals an unacknowledged prejudice towards single mothers/divorcees that may be routed in their catholic heritage.
I think because her name is likely not on the current deeds, ILs assumed that she had no claim on his home in divorce or upon his death. However, as his wife, even if he dies intestate, she’d get 300k or so and the rest would go into a trust for his children (but not the child from his first marriage, though); in a divorce she would likely get a decent settlement, but clearly that is not an issue for the OP.
However, if they now buy another house and her name goes on the deeds, then in the event of his death, she gets it all and can distribute the assets during her life and upon her death as she sees fit (ie, she could share evenly between all her children). I think this latter is their fear. And is deeply unkind. They are at perfect liberty to set up trusts and apportion their own assets for their grandchildren directly - and exclude the step child who may get an equivalent gift from paternal grandparents/father (or not).
They have absolutely no rights over the money they gifted DH or his siblings, unless at the point of gifting it they requested it be put aside for any children and put this in writing (which would be unlikely and bloody daft as none of them had children at that point). I really hope OP’s DH reads his parents the riot act and reduces contact with his parents until such time as they proffer a sincere and heart felt apology. I also hope his siblings are equally horrified - but there is the chance they feel the same way if they are devout catholics or more aligned in their values than OP’s DH, who has clearly pissed them off by not marrying some vestal virgin. 🤦🏽♀️