I presume that you already have a Durable Power of Attorney (and Health Care Advocate) issued in your name as it relates to your wife? May I recommend that in addition to your Last Will & Testament (LW&T) - or instead of, as the case may be - that you invest in a Revocable Living Trust (RLT)? With that RLT you can incorporate your LW&T, set up the appropriate powers of attorney, set up a trust for your sons (and your wife, if you die first) so your MIL cannot touch those funds, etc. Your situation almost demands that you go the RLT route for maximum protection . . . with the added bonus of no probate court provided that you also properly change ownership and/or beneficiary names on all assets, real and personal.
While a RLT costs a tad bit more than the ho-hum LW&T it's well worth the investment, and a competent Estate Attorney will be able to guide you and protect all your assets - real and personal - from the MIL, etc.
Good luck!
- SL