Since I never went more into this
1) they are about the same age
2) both have powers that involve the construction and manipulation of flesh
3) Amy evidently inherited her shard from her father
4) despite his policy of not harming women or children, Amy's mother did not trust Marquis with a child, and therefore kept her until she could not.
5) we know that Rachel lived with her mother until CPS was called when she tried to make rice on her own
6) nobody who would willingly enter a relationship with a villainess mastermind is completely stable
7) Dogs are not particularly nomadic creatures, so where Rachel triggered is probably somewhat close to Brockton Bay (within nomadic distance, in the same general area)
So I suppose that Rachel's Mother was in love with Marquis and visited him often, but kept Rachel unknown, not to keep her safe, but to use her as a trump card if she ever was in danger of being cut off from him, because until then she did not want to share his affection.
Now it could be that she kept Rachel away to keep her safe, and being absent from the home had something to do with Marquis's capture.
Also, wildbow has mentioned that Rollo wasn't a wolf or a dog, which could mean halfsies, or could mean coyote, which are not as prevalent in the new England area as they are in the south and south west.
Still I do believe this theory holds water