Hi Sam, welcome. I think most researchers would agree that letters and family bibles are acceptable sources. (Those who don't will surely let us know!) If you can find actual records that agree with the data in them, even better. If you find contradictory or inconsistent data somewhere, that should be stated and explained in the biography.
In my experience with a family bible, the dates are accurate, but you can't take it as an authoritative source for the spelling of names. And if it doesn't violate anyone's privacy, you should include info in the source citation about who is in possession of the bible or letters and whether you have actually seen the material or are relying on someone else's report. Just my $0.02 worth on the subject.