The issue I think is not the division, which seems reasonable to me, but the question of whether or not it is guaranteed.
If the OP's father's will leaves everything he owns to his wife and they have an agreement that she will then divide her money in a particular way, it is worth nothing. The OP's stepmother could change her will and leave everything to her own children, writing the OP out. That would be unfair, and morally wrong, but there would be nothing to stop her.
The rental property is solely legally owned by the OP's stepmother, so it is her will that is the issue. Whether she leaves a share to the OP is entirely up to her.
As far as I am aware (and I'm not a lawyer), the only way to guarantee the OP will get a share of her father's property is for him to leave her a share in his will and give his wife a life interest in it. He does need to rely on his wife's goodwill.
This is what we have done. We have severed our joint tenancy and become tenants in common. If I die first, my half of the house goes to our two children, with my husband having the right to live in it until he dies. If he dies first, his half of the house is left to his three children, with me able to live in it until I die.
Similarly, any savings in my husband's sole name will be split between his 3 children. Savings in my name will be split between my 2 children.
Essentially my stepson gets 1/6 of the total value (one third of his father's half). He will inherit less than my children, but I don't see that as unfair. I suppose I might feel differently if my stepson had been in my life as a young child and we had been close, but that is not the case. My husband is treating his three children fairly. I will not be leaving anything to his ex wife's child. And obviously his mother will not be leaving anything to my children either.
For what it's worth I agree with the OP that openness is helpful and we have discussed our wills with all three children (all of whom are adults).