Ignore the wind up merchant.
A: 75% between 2 joint DD’s, 25% between 3 DD’s
B: between 3 DD’s
C: between 2 DD’s
D. Between 2 DD’s
Inheritance from your family between 2 DD’s.
House A: You paid 75% of the deposit and you pay 75% of the mortgage - why would you give his DD 1/3 of that? Madness. Your girls share that and the three share their Dad’s 25%. Doing anything else takes away what belongs to your daughters. His eldest daughter has her own Mum.
DH has 3 children, so his goes 3 ways. You only have 2.
His eldest DD has her biological mothers share (whatever that is, is irrelevant) your joint DD’s have their mothers share. Would DSD’s mother be including your Dd’s? No, so why should you include her DD?
I would feel differently if she didn’t have her Mum.
Don’t be afraid to stand up to DH on this. HE has THREE daughters. YOU have 2. Your DSD has her mum. He’s a CF to think YOUR money should be given to his ex wife’s daughter. Ask him how much YOUR daughters will inherit from his ex wife?
It is irrelevant whether his DD inherits a huge fortune or a packet of peanuts from her Mum. It’s not your duty to even it up as it wouldn’t be his ex wife’s duty to share with DD if she had lots of money.
I would have this locked down because I get the feeling that if you got knocked over by a bus tomorrow your DD’s would, at best, be sharing everything 3 ways and at worst with other children if he remarried. LOCK the other houses down so your DD’s get B&C at age x and their share in A&B guaranteed.
Do not blindly trust him to follow your wishes.