What song by the Beatles was written about Mia Farrow's sister?

Dear Prudence was a classic Beatles song! Although it was written and sang by John, Paul and George had integral parts in it. I love the way they were still working so well together at this point. I believe Ringo was off on Holiday when it was recorded. 😁

I think I saw a TV programme about this and they were all staying at the same Ashram in India when they composed this song.