Families like this are just sooooo not worth it. I had a mom once try to make me feel guilty for taking vacation ( even though I gave like 5 months notice). I just changed the subject quickly, basically ignoring her whining and she stopped and never complained again. There is no way I am going to explain myself or worse apologize for taking time off, and then feel sorry for them because they didn't find backup. If they dont have backup, NOT.MY.PROBLEM. although they know from the get go that I don't have backup.
Honestly I think that having a "backup" has done nothing but confuse your clients because you say sometimes she is there, sometimes not...if you don't call on her that often is it even worth it?