Does anyone Co-Sit with a family member?

Does anyone share sitting duties with family members or another sitter? Do clients have a problem with this?

Trying to be upfront with a new client, I told her that my brother often comes with me to help. She informed me that for liability reasons, I was the only one allowed on the property. I understand that, but it has gotten me to wondering if I should never let a family member come with me on future sits? Am I wrong to share duties with them? There have been times when I didn't tell people that someone else would be with me, which I realize is NOT professional - so now I am trying to be upfront about any other people who may help me out, but should I never let someone come with me? Or, as long as I inform the client upfront, is it OK?