We recently went to trial on a matter involving five siblings. My firm represented three of them, one remained neutral and the other ("Monica") was our opponent.
Following the passing of their Mother, Monica produced a will, trust and quitclaim deed showing that she was the sole owner of Mother's house. Those documents were challenged by my clients on the basis of undue influence.
To be successful, we had to prove that Monica procured the 1995 Will through acts of undue influence. The document was executed 13 years prior to the hearing. In most cases, the delay would have presented an obstacle at trial because memories tend to fade over time. But not in this case. My clients saw this coming from the day Monica moved into Mother's house (in 1992).
So what did they do to protect their inheritance? They kept journals. Each of them kept separate journals and made entries every time they suspected something was not right between themselves and their Mother, Monica and their Mother and between themselves and Monica. It was their individual factually specific journal entries that helped prove the undue influence and invalidate the 1995 Will.
When someone asks how they can protect their inheritance in the least intrusive manner, I often tell them to purchase a personal journal from a stationary story and make very specific and detailed entries regarding any unusual behavior involving their parents (or benefactor) and any other person--especially if it involves money.