I doubt Jill needed to use Clomid, she is just a fertile woman. Back in the day, women married as teens and had a lot of pregnancies right up until they went into menopause. Jill is not an outlier.
As others have noted, women who marry young and don't use contraception have historically had multiple pregnancies. Alexander Hamilton's wife, Eliza, had 8 kids as well as a miscarriage between the ages of 25 and 44. If you know their story, they spent large chunks of time apart as he served in the Revolutionary War and was Secretary of the Treasury, otherwise she'd have probably made double digits easily.
Henry VII first wife, Catherine of Aragon had 7 pregnancies in 9 years although only one child survived. Her other pregnancies ended in miscarriage, stillbirth or infants who died shortly after birth. Unfortunately for her, she went into premature menopause by the time she was 40 or there would've been more.
The chance of twins for people not on fertility drugs is around 1% but it rises as a woman ages and as she has more pregnancies. Fraternal twins can run in families on the mother's side, but, even then, the inheritance is multifactorial and the risk is around 5%. My partner currently has a patient in her mid 20's who is carrying twin girls. She has twin boys who are two. No fertility drugs involved other than maybe a glass of wine.