There's a bigger picture at play to be honest.
The DF and DM worked and paid off their house so they were mortgage free. Maybe DF paid the lot. They both worked together, regardless, to own their home and not have to scrape by with mortgage payments in their later years.
DM has sadly passed, and presumably OP is the only child, so she's left her half to DD. I presume DF's will says the same.
What's unusual here, is that the remaining parent usually stays in their home until they pass themselves, or move to perhaps a care home, or downsize to another property. It is highly unusual for one parent to die, and the adult child to declare immediately that half the house is theirs so they and their whole family are moving in. Presumably at the time DF is mourning the loss of his wife.
I can't imagine one of my parents passing and me grabbing "my half" and moving myself in. That's my mum's/dad's house with one of them still living in it! Similarly, I would be quite ashamed of any of our children, if DH and I, worked all our lives to pay off our beautiful home, I lost DH at a reasonably young age, and they turned up on the doorstep, moving themselves in, "it's technically mine as well now, so move over mum, oh and I know you'll be several hundred a month worse off because of it, but it's mine, so...."
How this arrangement came about, I think is critical to how this loss of money issue is tackled. If DF wanted them desperately to move in, and this was an oversight on his part, it's very different to "we're living here, and you can't stop us because mum didn't put the right clause in her will"