"Commingling is a very, very common litigation issue, whether it comes up in divorces or in post-death litigation," says L. Paul Hood Jr., a former practicing estate attorney and co-author of Estate Planning for the Blended Family.

The problem is, in trying to safeguard an inheritance, the parents may set so many conditions that it disrupts the marriage, when a couple is supposed to be pooling their resources to build a life together, says Emily Bouchard, a managing partner at the Wealth Legacy Group in San Diego.

"A husband may not even have the freedom to buy a home with his wife's name on it," Bouchard says.

Bouchard worked with a couple that had been married for more than 20 years and had four children. The husband's father, who'd made a lot of money, always treated the wife like an outsider. She was made to sign a prenuptial agreement, she was left out of the will, and in the end-and unbeknownst to her until recently-her name wasn't even on the deed of her family house because trust money was used to purchase it. When she found out, she was devastated, Bouchard says.

"She felt like she was not getting credit for raising the children or contributing to the household," Bouchard says. "And it was poisonous to the couple's relationship."

In extreme cases, parents will be so desperate to cut their in-laws out that they cut their children out of the will and leave the money in trust to their grandchildren. If the grandchildren are minors, they will have a third-party trustee oversee the assets. The trust documents may say a certain percentage of the money, say $5,000, can be removed each month, but anything over that would require consent of the independent trustee. But these situations are rare, says Donald DeLong, a partner at Brickley DeLong P.C., a CPA firm in Grand Rapids, Mich. DeLong says just 5%, or about a dozen of the estate plans he's done in the last 27 years, have looked like that.

"They want to make sure the gold digger doesn't get her hands on it," he says. "But it's rare because essentially what they're doing, by cutting out their in-laws, is hurting their own children. Their children don't get the true enjoyment of their property."

Sometimes, the parents are being unnecessarily paranoid. Everett Sussman, an estate attorney in Shelton, Conn., says he recently worked with a matriarch who said she loved her son dearly but, as she put it, "He married an idiot." She feared if her son got divorced, his ex-wife would remarry instantly, and she was concerned her money would not reach her grandchildren.

"I know of nothing the daughter-in-law has done that's evil or criminal. The mother just doesn't trust her," Sussman says. "But she was not going to trust anybody. No one is going to be good enough for her boy."

To that end, the mother created a trust that gave some of her $2.2 million estate to the son and his sister and the rest to the grandchildren through a generation-skipping trust. The daughter, who was a stewardess, was put in charge of the money while the son, who was actually in financial management, was given only limited power over his assets. That way, the son's wife could not get at his portion.