Zelretch took Nagato Tohsaka on as an apprentice, to teach him Magecraft (NOT True Magic). The Tohsaka crest took part of the Edelfelt crest at some point, because Rin's great-grandmother on her father's side was one and passed it on to Tokiomi's grandparent/parent. At least, that's how I remember it.
Was refering to how new/branch families get crests. Either an old one buds a blank crest for them or they need to get a phantasmal beast part to create a new one.
Odds are good Zeltretch lent them a starter.