14Princess Peach is Bowser Jr.’s mother
Bowser Jr., the son of Bowser, first appeared in the disguise of Shadow Mario at the beginning of Super Mario Sunshine. He frames Mario for spreading graffiti across the island, and kidnaps Princess Peach. Finally revealing his identity and claiming that Princess Peach is his mother, Bowser Jr. says the
"evil" Mario is the one who kidnapped her. He attempts to bring her to Corona Mountain, where his father is relaxing. It could be that Bowser told his son a lie in order to convince him to kidnap the princess and pass his hatred for Mario onto the next generation. However, Peach never denies these allegations, and it is never even hinted at another person who could be Bowser Jr.'s mother.
There were also seven Koopalings in Super Mario Bros. 3 (1988) who were believed to be Bowser's children. In an interview with Kotaku, Miyamoto commented, "Our current story is that the seven Koopalings are not Bowser's children. Bowser's only child is Bowser Jr., and we do not know who the mother is." So even the creator doesn't deny Princess Peach may be Bowser Jr.'s mother. Though that does make you wonder where the rest of the Koopa kids came from...