Any advice

TBagpuss

If they were separated it would be pretty standard to sever the joint tenancy. Equally, it's pretty common for a person to update their will at that time as of course most separated / divorcing couples don't want to benefit their ex.

It's a bit odd that your dad left everything to his step-son's and nothing to his own child(ren) -or were you on bad terms with him?

IF your mum makes an inheritance act claim one of the things a court would consider would be what she might have received on divorce, and also the effect of any successful claim on the other beneficiaries.

Assuming the step-sons are adults, mum's housing needs are likely to be a key factor .
 I second the advice to get legal advice asap. 

How much us the house worth? Would the value of your mum's share be enough for her to afford to buy somewhere smaller, if it was sold? Did he have any other assets as far as you / your mum know?
The executors reluctance to provide a copy of any medical report is a red flag as you'd think it would be in their interests to produce it if it shows he had capacity...