Basically, it's a 'reveal' that the embroidery that has been done by the family went from spirit-blessed way (like the founder and Ewey do) to this 'contract with monster' way at some point.
The spider, which is normally a monster, is obedient to the family via the contract.
So the person in the background is either Ewey's sister or mother, forced to sign the contract and is slowly dying from the effect of the monster spider possibly draining their life to replicate the blessing (since I think all the spirits left with Ewey at this point)
Btw part of the contract is also that it is broken if the person is no longer part of the Nylle family.
So the father thinks that when Rodan marries Ewey (his assumed reason for Rodan taking her), the contract will be broken and Ewey's little white spider will return to being a monster and kill people nearby, Rodan and Ewey included.