So how could this situation have been avoided? I'm guessing your relative was simply following the advice of his own estate lawyer. Sort of like how an amicable divorce degenerates after the lawyers get involved.
The evil in this case was IMO premeditated. I suspect that one day he realized that if his mother died he wouldn't have her house to live in nor her savings and pensions to feed and clothe him. He rarely worked so he spent his time on his computer in his mother's basement researching solutions to his self made problems. With no income his reasonable share of the estate wouldn't buy him a house or the status he thought he deserved.
The biggest problem was denial on his mother's part. Being "Old country" the males were ranked higher. She was very concerned about family image and was embarrassed that he didn't have a job, as if it was her fault. My M-I-L was elderly and on heart meds so no one wanted to stress her by bringing in outside help. "He promised"
The family reform act of years ago made allowances for people made dependent by others. Those considerations can be misused. Final decisions in court cases are made by judges and they can be influenced by personal feelings. Behind every nasty lawyer is a nasty client. If all a person wants is a fair settlement that's how they instruct their lawyer. Some people need revenge and instruct accordingly. Court isn't as predictable as putting together Lego blocks.
If everyone had a crystal ball and knew the detailed future there would be solutions.
A lot of people think that what a person writes in their will is iron clad. It isn't. Today you could sell your motorcycle, use the money to buy premium single malt and give it out to the winos on skid row. If you put it in your will that the proceeds of the sale of your bike went to scotch for winos it would probably never happen.
If you sell your assets while alive and cognizant and give the money away there isn't a lot to fight over. The problem is how long will you live, how much will you need and who do you trust to do the right thing if your situation changes?
If we had suspected the extent of the problem I could have purchased the house, allowing my MIL to keep living there as long as she wanted. The house was in a small town and only worth ~$450 K at the time. BIL could have continued to live there if he stopped acting like a jerk. MIL could disperse the money evenly while she was alive. Minimal probate, minimal lawyers, minimal real estate fees.
Who would you trust to do the right thing if you gave them all your assets? Would you trust their spouse?
What if your situation changed and you needed LTC at $8,000 a month? Full time nursing at a quarter million a year?
Honesty is the most important trait a person can have. Laziness, low intelligence, naivety, physical weaknesses are predictable. You can compensate for them but if someone wants to stab you in the back you never know what they'll use or when.
In general, I suspect another problem with inheritances. They are seen as windfalls, something you didn't work for. Therefore you should expect to share the wealth, after all, you didn't really work for it. You're walking with a friend to the diner down the road and while buddy is watching the sky you're watching the ground. You see a $50 bill and pick it up. Suddenly you should pick up the tab for lunch because you're lucky.
The topic is wills but estate planning is more important if there is the potential of conflict. The hard part there is that ethical people are usually reluctant to bring up the issue with the testator for fear of being seen as a greedy estate monger. Life sucks, death is worse.