Scholastica, sister of Benedict, Abbess of Plombariola, c.543

The most prominent stories about Scholastica seem to be that she was a very devout child and brought her brother to faith, and that one of her last acts was to try to prolong a religious discussion she was having with her brother. She seems therefore to be in the category of "Sister of the more famous...". I think my feminist roots are showing here, as I can't help feeling that if it hadn't been for her, Benedict wouldn't have been particularly famous, and I suspect some androcentric bias in the history books.

A very interesting and slightly related story (see link in title; more here) is of the St Scholastica Day Riots in Oxford. This was an early instance of Town vs. Gown, starting with a row over beer and ending up with nearly 100 dead. These days the row would still be over beer (or possibly alcopops) but the protagonists would be slightly less likely to be armed.