Hello and thanks for the extra information.
Unfortunately, your mother-in-law's will has no effect on what happens to her flat - because she is still alive. A will is only bindoing after the person's death.
I understand that the flat is owned in the joint names of your late husband and his mother who is still alive.
You don't say what form of joint ownership they had -
(a) If it was as beneficial joint tenants in equity, then when your husband died, his share went automatically to the other owner ie your mother-in-law. In other words, on your husband's death, she becomes the owner of 100% of the flat. What happens on her death will depend on what she puts in her own will, or without a will, whatever the laws of intestacy are at the time she dies. But before her death, as she owns 100% while she is still alive, the care home will take the value of ALL of the flat into consideration. Her will is not binding until after her death.
(b) if your late husband and his mother owned the property as tenants-in-common, then your mother-in-law does NOT automatically inherit your husband's share of the flat - it goes to whoever he specified in his will.
If your mother is eligible to go into a care home, the local authority will assess her means - both her income and her capital. The capital includes the value of any property or share of property that she owns. If her capital or her income is above the limit set, she willl have to pay part or all of her care home fees.For the financial year 2014/2015, the upper capital limit for the means test in England is £23,250. The value of property is ignored for a temporary stay in a care home.It's also ignored for the first 12 weeks of a permanent stay.
So - if the flat is now wholly owned by your mother-in-law because she automatically inherited your late husband's share, the whole value will be taken into account in means test. If the flat is still jointly owned, then only her share will be taken into account.
If you are living in the flat, then as a relative of your mother-in-law, the value of her property will be disregarded.
How the property is valued, and other issues concerning the local authority's discretion to disregard it's value are complex - for details, please see Age UK's Fact Sheet 38 - here:
I hope this helps and I wish you the best of luck.
Thanks and best wishes...