Midwife Nikki replies:

Quick or ‘precipitate' births as they're also known, last an hour or less, progressing rapidly from ‘no contractions' to the delivery. It may seem better as it's over swiftly, but there are drawbacks. Longer labours allow your body to release feel-good hormones called endorphins, to ease the pain.
With a precipitate labour, contractions are immediately painful, and often every two to three minutes. This can shock both mother and baby and there's a risk the baby might be born distressed.
Your sister's second labour may well be fast too, so she needs to discuss her potential problem with her midwife.