It sounds as if you've done a good job in getting your mother's affairs in order.
Wills are actually quite difficult to contest. Your mother's husband would have to prove either that your mother did not have the capacity to make a will because she was not mentally competent to do so, or that she was put under undue pressure to make a will that did not represent her true wishes.
Both of these issues are usually hard to prove. But to be on the safe side, you would want to be able to document that your mother was competent and not under any pressure.
For example, if your mother is still competent, it might be a good idea to have her videotaped stating that her current will reflects her wishes.
If your mother hired an attorney to help draft the will, you should also consult with him or her to see what evidence there is of your mother's competency. Ideally, the attorney met with her alone to ascertain that she was not under pressure to make the will she did.
Be aware, however, that your mother's husband will most likely be able to claim an "elective share" of your mother's estate, regardless of what her will says. In most states, a spouse who does not inherit under the will of their deceased spouse may claim a share ranging from 1/3 to 1/2 of a deceased spouse's assets, instead of whatever the will left them. This varies from state to state and can also vary with the length of the marriage and the number of children the decedent left behind.