There is no such thing as a joint will, there are mirror wills, where a husband and wife can make wills that mirror each other but there is nothing stopping one of them from changing their own at any time, which seems to be what has happened with your SM, once your dad died.
It is likely that, if her own children are the only beneficiaries, that they will also be the executors of the new will, not your brother.
It is disappointing if you have been cut out against your Dads wishes, but that is your dads fault, he could have left his half of the house to you and your DB in his will, to ensure you weren’t cut out on his death. It is a very common problem with step parents, from MN threads. DH and I have mirror wills but I do wonder whether I should leave some of my money direct to my DC in case he re married. Sad to have to think that way, but I would ensure that my DC were provided for in my will if I was widowed and re married.
A will can be held by anyone, it doesn’t have to be written by or lodged with a solicitor. Maybe your SM never had a will with the original solicitors, or requested it back and made a new one that is held by her children?
I would guess that your first port of call would be her children, as they are her next of kin.