My advice is speak to your Dad. He is your Dad right and he raised you, loves you and you want him to be happy right.
Respect his new relationship, why should he not have companionship and no one is ever going to replace you and your mother, but we all want to be happy.
Go see a solicitor together, your father needs a will.
And see an accountant, for example you use the banks money to buy cars and houses - so using a proportion as a sum as a deposit, some to buy things and saving for your future and for looking after your Dad as he gets older are all important.
You need to look after him as he becomes older and vulnerable just as he looked after you and kept you safe when you were a tiddler.
He is your Dad you can speak to him about anything. Try it.