As your mother is LA funded, the most she is likely to leave in her estate is £14,000. (Should her assets fall to this level, then the LA stop taking contributions from her, the Top-ups would continue.)
In order to have these family Top-ups repaid, there would have to be sufficient funds in the estate, after the funeral costs.
As first party Top-ups are not allowed, if the person does not have enough assets to fully pay for their Care, then they do not have any money for Top-ups.
Top-ups have to come from a third party, family or friends, who are prepared to meet this cost for as long as needed, and the increasing cost.
I do not see how this Top-up cost could be recovered from her estate.
At this point she does not have the capacity to agree a debt, that could be paid from the estate.
The LA could start a claim for Deprivation of Assets, where they would claim back the fees they have paid on her behalf.
Step Dad could agree to refund this top-up cost from the sale of the house, when he passes, subject to him not requring to sell the house to fund his Care. (and him out living mother!)
This does depend on the ownership of the house.
Bod