Answering the question @cambridgecoral was actually asking without any kind of moral/ethical judgement:
No, it doesn't really protect your interests at all.
If you can afford to get a solicitor or accountant to look at it, or if you're good at numbers you can set it all out and present it to him to see if you can persuade him to change his mind.
Main issues I can see are:
Can you persuade your father to buy the house with the new partner as tenants in common, in shares relative to their contribution into the property?
I.e. from what you've said, about 66% him/33% her.
You might try to sell it to him as protecting his interests in case the relationship doesn't continue, as otherwise if they buy as joint tenants she's initially presumed to own it 50/50 with him.
Having a deed of trust around the property ownership helps, but at least make sure they're tenants in common in unequal shares.
You then need him to make a Will which:
a) covers any issues about "anticipation of marriage", so that the Will isn't invalidated if he does marry the new partner
b) leaves his (66%) share of the house to you and your brother
c) (possibly) leaves the new partner only a life interest in the house, so that she can live there after he dies but cannot sell it or use it for her care fees if needed.
If he's as nasty as you say, you could probably also get him to put in provisions such that if she has a new partner/marries/ co-habits with someone after his death, then her life interest in the property terminates.
See a solicitor to get full understanding of all the above.
Also be aware that Wills can be changed at any time, as long as the person has capacity.
Offering no moral judgement on the situation, just some practical steps to try to safeguard your father's assets for you and your brother.