Nice, you and your wife certainly have a potential mess heading your way.
I recently completed a three year stint where the wife and I were executors for three parents of ours. My mother's case closely resembled yours. I have two siblings, my sister is a train wreck caused by drugs, alcohol, and a lifetime attached to the welfare nipple. My brother was a financial disaster with no job, no money, $125K in unsecured debt, and the sheriff at his door, "helping" him move.
Where the situation differs is that my Mom divided the assets evenly, with the requirement that I step in and fill her role as chief enabler of my totally screwed up sister. The language basically directed me to assume her guardianship, which was my Mom's delusion, as this loser desperately needs a kick in the ass, not a guardian. Since I refused to continue playing the game with my sister, and even told my Mom that I refused to be executor if she insisted on this madness, I was stuck in a rough spot. In the end, my lawyer convinced a special needs trust to take possession of her funds, and distribute them to cover her living expenses, until the funds were exhausted. Since the trust was of a similar mindset as I, when it came the fact that she was a pretty weak candidate for being "special needs", they only accepted her after my very savvy lawyer did a lot of begging.
MY brother took the money ( about 1/3 of a million) and cleaned up his financial mess. He then bought a nice little home, for cash, and continued his life, living as close to being worthless as it gets. This continued for a few years, until he decided that he had enough of being a loser. He is now back in sales, and worked his way, quickly, up the ladder. He is about to be promoted to a region VP position in a legitimate marketing outfit. He should see his first six figure 1099 in 2015.
I guess the point of this is to not give up on the bro. with PTSD. There are ways to carefully dole the money out, if he continues to crash and burn, through various trusts designed to do so. And who knows? Maybe, one day he will snap out of it, and get back on track again, like my brother did. I would of bet $10k that my bro. would of hit 50 yrs. old as a minimum wage, working drunk, who was barely holding onto his house, and still driving a 25 year old Suburban with shit falling off it. But, happily I was wrong.
As for your remaining SILs, I think you need to give a LOT of weight to the fact that the MIL has no relationship to them anymore. IMHO, people tend to over value biology when it comes to who deserves what, in settling an estate. If the MIL wishes to not give anything to offspring that she no longer is involved with, that's her wish, and it needs to be honored. As an executor, taking your portion and redistributing it, based on what you think is "fair" is a bit troubling. Is it happening out of a sense of guilt? Is it happening because there is some need for justice, balance, or making things "right" even though this specifically disregards the wishes of the deceased? IMHO, i's a bad road to head down. The Mom might be perfectly fine with the knowledge that giving either of these two a dime is only going to enable them to continue a downward spiral, so what gives anybody the right to disrespect that.
I had clients in the past that owned several businesses, and had a high net worth. They had a daughter who worked her ass off in the family businesses, and a son who was a serious drug addict and gave them decades of grief. In the end the son got nothing, and the daughter got it all. It blew the son's mind to know that he got stiffed, but he couldn't see the fact that he would of destroyed everything they worked their wholes lives for. Being fair in that case was not giving the son enough cash to be buying coke by the kilo, or giving him a business to run into the ground, particularly since they set him up with his own business in the past, and he threw it all away. Life isn't fair, and sometimes cutting the pie equally is the worst move of all.