It's not about thinking anyone is making a mistake. The Grandma is doing what she thinks best with HER own money – and actually OP is coming off as grabby & controlling re: her MIL's money and love for other people like DGC.
It's about accepting that real life does happen, just like her DC's previous divorce. While I'm sure plenty work out, statistically, the majority of second marriages after divorce end badly! But even without that, death etc could happen. With other figures (OP, OP's adult birth children) introduced into the inheritance mix, the best way to ringfence the money for her DGC whom she loves is to will directly to them.
I'd argue that this is a way for her not to interfere in DC doing whatever he wants (not that she should be interfering but ykwim), while still guaranteeing that her DGC are taken care off after she's gone from this earth.
The assumption in doing so is that DC is already happy, in terms of solid property ownership and buffer money. He owns a property with the woman he loves! Plus has sale proceeds from half his previous home.
If OP or OP's partner needs the grandmother's house to keep them happy, frankly I'd be suspicious of their motives – the house might not make it to DGC in their hands. All the more reason to will directly to DGC.