This is very difficult when one member of the family sets themselves up as guard dogs over the dying one. Is there any other member of the family that you could appeal to come with you? Bullies will often back down when faced with superior numbers.
Remember also that her partner is facing losing her and may well be having his own mental breakdown.
Failing all else, I would go and if he didn't let me in I would call the police and ask them to do a wellness check. That you are worried your stepmother is dying and not getting the care that she needs.
I don't think you can do anything about the bequest, if it's not in the will and she hasn't sorted it, best not to think about it.
My sister wanted to give me a gift but she also hadn't included me in the will, so it came to nothing.
Sending my sympathies to you in difficult times.