Do read this labnotes policy published by Oxford University. Many points on good practices for keeping a lab notebook while working on patents, not all will be helpful.
It suggests that "entries should be witnessed and corroborated by someone who is not working on the same project but who is competent to understand the work."
I think the logic behind this suggestion is that an ordinary person skilled in the art is vouching that you did that technical work on this day, whereas family members attestation cannot be considered equivalent to that. They don't understand your technical work.
As you'll get many good suggestions from the above cited link, but from law's point of view, you should be concerned about logs, what work was done on which date while working on the invention. It is important for novelty aspects of the invention and inventor's credit/authorship, which means what is claimed by whom in case more than one inventors are working on the same project.
On second thoughts, whoever will be signing your logbook will be an indicative of the fact that he received that particular knowledge from your logbook. Another suggestion will be to keep a record of people whom you will show your logbook, get an NDA signed before you show logbook. For the witness purpose, this is what I've read at the end of disclosure, "I've witnessed and understand this disclosure. I agree to keep this disclosure confidential" or "The above confidential information is witnessed and understood by", afterwards was signature of witnesses with name, address & contact details.