Leslie is the middle child out of the triplets: her older brother Frank Jr. Jr. and her younger sister, Chandler Buffay. They had trouble conceiving naturally, so Frank asked his sister, Phoebe Buffay, as a wedding present, to be the surrogate of their child. Leslie was born with Frank Jr. Jr. and Chandler in the episode 'The One Hundreth.'
They were seen again, one year old, in the sixth season and last season, when they were five years old, Frank asks Phoebe to take one of them (like in 'The One Hundreth' were Phoebe asks Rachel to ask Frank Jr. if she can keep one of the babies), but seems to love them too much to give one away.
Phoebe Abbott is not Leslie's grandmother because her father Frank Jr and Phoebe Buffay have different mothers (both adoptive and biological. Lily Buffay has nothing to do with Frank, and Phoebe Abbott is Phoebe and Ursula's birth/biological mother, but not Frank Jr's. Frank Jr is the child of Frank Buffay Sr and an unnamed woman)
- No one seemed to notice that Leslie is the name of Phoebe's ex-singing partner.
- Leslie was actually played by a boy in the episode "The One With Joey's Porsche" (You can see this when Leslie was then found sitting in a drawer in a green dress without a diaper. You can clearly tell it's a boy.)