Yes, proper estate planning is essential to avoid such situations.
My aunt's (blood uncle's wife) father died and her mother inherited everything. It was apparently his wish that his surviving children eventually receive his estate but this wasn't specified in his will.
The mother remarried about five years later. She never got around to making a will. The mother died in a car accident about six months after the marriage. In the absence of a will the new husband inherited the mother's assets. The husband was willing to work with my aunt and her siblings but in the midst of negotiations he suddenly died of a heart attack.
Then the husband's estranged daughter, who no one even knew existed, popped out of the woodwork to legally claim everything. There was a lawsuit filed, but this daughter was the sole legal heir. The daughter was eventually persuaded to return some sentimental items and jewelry, but in the end wound up with the bulk of what was "supposed to be" my aunt and her siblings' inheritance.
When I die, if my wife decides to remarry, practically any guy would be an improvement over me.