It depends on the laws in your particular state, but in most cases, if you are still legally married then you have a right to part of this estate as well. If and when you decide to divorce after a length of time has passed during your separation, you should be able to use that estate inheritance as part of his total worth when determining your alimony and/or child support.
Check with an attorney in your state. Depending on the state of your relationship, you may need to be on the alert for ways that your husband may attempt to get around any claim you have on his mother's estate. You will need to consult an attorney who is well-versed in the laws of your state as well as the proceedings of divorce court.
This is a bit after the fact- but I just wanted to post this source of information as I am currently going through a similar ordeal and was looking for more information on the whole process when I came across your own question..
* Wills, Trusts and Estates - a great overview of the whole legal process.
* Wills Overview - From US Legal, has info on all aspects of Wills.
*Estate Planning Bits - Gives the top ten issues regarding wills, I found it to be a good read.
Hope these can benefit anyone else wondering the same questions.
My husband also just inhereted from his mother's estate. Unless your husband declares it in a will and if there are children what he determines in the will is what the spouse receives. Obviously it all depends in the state you live. My husband and I are from Puerto Rico and the determination is done through a will. What you share as husband and wife is separate from his inheritance. The best thing to do is to consult with a local attorney.