Practical post as I worked in Probate...
There are laws about how the possessions of someone can be 'disposed' of, even when there's a will left that specifies someone's wishes and what they want to happen. It's partly based on the monetary value of the estate but can
take account of what the deceased person wanted to happen on a verbal or tacit agreement.
If you feel strongly about what your stepfather may do, then think about taking legal advice to see if you can challenge the will or how the estate is wound up if there's no will (in fact ironically you're better 'protected' under law if there's no will as children will inherit from their parents as a legal right
if the estate is over a certain limit which is set in law depending on the date of death).
This is not something to enter into lightly as it really does cause bad feeling within the family of course, but you sound like you're very concerned, so it's worth looking into, even if you take no action in the end. You don't have to go to a solicitor straight away - the Citizen's Advice Bureau is excellent for this kind of situation and can take you over some things (like putting caveats on the estate which is a holding procedure so whoever dealing with the estate can't get a grant until any disputes are sorted out). You don't necessarily have to have a solicitor unless you really want to press ahead with a challenge but if you do then it's best to have skilled representation as court proceedings may be necessary and the whole Probate Law area is a minefield unless you know what you're doing...
I'm so sorry you have this horrible worry on top of everything else and I hope this can be resolved as painlessly as possible.