How related two conferences are if B is sponsored by A? If A is a top conference, does this make B a worth-trusting conference also? For example, SIGCOMM is a top conference in computer networks & communications, and CoNEXT is sponsored by it. Does that automatically make CoNEXT a quality conference?
The top conference has reputation to keep up, so if they associate themselves with the other conference this indicates that they trust the organizers of the other conference to do a good job. So, I'd see the endorsement as a sign that the level of the conference is probably good, although there are no guarantees.
I'm not sure what it means for one conference to sponsor another. In the particular case you mention, "SIGCOMM" is both the name of a conference and the name of a group within the ACM that sponsors that conference. So it's likely that SIGCOMM (the group) is sponsoring CONEXT, rather than SIGCOMM (the conference).