When you marry the love of your life, you are also taking on that person's entire family, and that might be surprisingly unpleasant. Franklin Roosevelt's mother Sara never liked his wife Eleanor, which made life hard for Eleanor while Franklin appeared oblivious to the tensions. Harry Truman suffered the continuing scorn of Bess's mother, who even moved into the White House with the couple. To his credit, Truman graciously memorialized his mother-in-law as a "grand woman."
In royal families, marital relations can lead to power grabs, paranoia, and death. Herod the Great of Judea felt threatened by his second wife's family, and had his brother-in-law, wife, mother-in-law, and two sons killed, although not all at the same time. It was business as usual for the ruler who decreed the death of all boys under two just to get the one who had already fled the country. Read these stories and others of families you really didn't want to marry into at Mental Floss. You can also listen to the list in a video at the same link.
(Image credit: Alexander Master)