I'm not seeing the "conditional acceptance" part of the OP's story; admit that could make a difference.
Other than that, though, the OP can cancel the gig and probably can do so in good conscience because his family situation is more important. It is, truly.
That does not change the fact that he is the one who cancelled, and the bad is on him. Any thoughts about this somehow being the other bandmates' fault because the gig wasn't good enough to keep is either immature or delusional. Once you agree to them, they're all the same.