Let me answer this as far as I am able with the limited information I have to hand.
A lot depends on whether the sale by your mother to your sister was a genuine sale for money or whether your sister coerced your mother into doing it.
Assuming that it’s a genuine sale than your sister is now your stepfathers landlord in exactly the same way as if she was not your sister.
Your mother had a tenancy at will but we don’t know whether she paid rent or not. Your stepfather have the automatic right to live in the property by virtue of it being the marital home but that only applies insofar as there is in effect occupation by the married couple as a occupier/licensee/tenant.
Your sister is now your stepfathers landlord and if she wants him out and she’s going to have to give him notice although the notice period changed on 1 June to 4 months.
I’m sorry, I wish I could give better news. Your stepfathers fairly and squarely in the hands of his landlord, your sister.
Thank you for letting me assist you with your legal question. I am glad that I was able to help.
I am not certain whether that answers the question for you or not, but I am happy to answer any specific points arising from this.
It will be my pleasure to help you again either further with this or any future questions you have