100% this. He's creating a future shit storm OP. Nothing causes family rifts like perceived unfairness in inheritance.
If he isn't an arsehole who's going to be enjoying laughing up from hell at you all squabbling and potentially seeing you inherit nothing at all OP, then he would do better to leave people a percentage of his estate. That way it doesn't matter how much is left when he dies because a percentage doesn't change regardless of the value of the estate.
Then everything gets sold, the inheritance tax is paid from the whole and the percentage divvied up according to the will.
If you don't want to be left a house with someone having a lifetime interest in it (and bear in mind your DM could remarry a total dickhead who trashes the place or develop dementia and burn it down accidentally, although at least insurance would pay for that second one), then perhaps he should just leave the house to DM and if it gets spent on care fees so be it. You'd have had your percentage of the inheritance from DF already. So anything DM left you would be a bonus.
If he hasn't made provision for DM living costs then she could downsize, invest the capital and use the interest from that to supplement her state pension. She'd also have access to the capital to spend in an emergency, like if she's living in a block of flats and the management company decides it needs re-roofing she'd just get an invoice for her share. Small flats are doable on a state pension, although it would be tight depending on what the monthly management fees were. Not a retirement flat, she won't be able to afford the fees on that on a state pension, just a regular flat. Or downsize to a small 2bed terrace, invest the leftover equity as before and get a lodger for the other room too, to top up the state pension. More council tax and heating costs but the lodger will offset that. You'd have to ensure you charged enough to take into account the extra utility bills from an extra person.
Lodger's have few rights so evicting a bad one or when you need to sell up is easy, no court proceedings, just one week's notice and change the locks. Don't put a lock on their bedroom door though, that's fundamentally against being a lodger and could stray into tenancy territory. Your mum could have a lock on hers because it's her house so it's ok for her to lock people out of part of it.
He also owns a narrow boat, which he is leaving to another cousin.
Which would end up sold for inheritance tax, no doubt.
He owns a third house, which he bought years ago for a distant cousin to help her out. He says there is large mortgage on this house and not much equity, and he is leaving this house to her.
She'd get the equity. Unless there's some life insurance to pay off the mortgage if he dies. Until that mortgage is paid, it's not his house, it's the banks. They'd sell it if they repossessed it (do they do that when someone dies?) or the executors would have to sell it, to recoup the banks losses. If it's repossessed I'm not sure cousin would get anything at all. If executors sold it bank would be paid and cousin would get the equity, minus selling expenses. You can't transfer a mortgage, so cousin staying put and paying mortgage isn't an option (I'm assuming this mortgage is in DF name). She'd have to get her own mortgage (sounds like it won't be a possibility, else she'd have done that in the first place) to remain living there, effectively she'd buy the house from DF estate. Or the house is sold, she takes her equity and moves out.