I'm a willwriter and also do lots of Lasting Powers of Attorney. PPs are correct, there are two types of LPA, one for Property and Financial Affairs and one for Health and Welfare.
In the circumstances, I would expect your father to appoint his wife as his Attorney, failing which you and your brother would be Replacement Attorneys jointly. I would expect his wife to appoint your father to be Attorney with her daughter as Replacement.
You are going to have to tell your father how you feel, and as you have to consent to be named as Attorney, whether primary or replacement, you don't have to sign anything you aren't comfortable about, although you can't of course make him appoint you higher in the rankings than your stepsister.
It sounds as though your father may just be going along with this to keep the peace but I agree with you that you and your brother ought to have LPA before your stepsister but after your father's wife.
I'm also slightly concerned that they seem to have made mirror wills to each other on the presumption that the survivor won't make a new will (or remarry which revokes an existing will) which would have the effect of cutting out the children from the first marriage on either side. I normally advise clients in stepfamilies to consider making life interest trusts which ringfence at least each spouse's share in the home for their children.