The second part of your sentence may be harder to understand because of the two consecutive verbs, one for the relative clause ("who I know") and the main verb ("[no one] knows").
Also, when I try to read your sentence out loud, I tend to pause after "I know" to signal the end of the relative clause.
Here's an alternate wording:
..., but I don't know anyone who knows about her.
This attaches a relative clause to the object (instead of the subject), allowing the clause to be at the end of the sentence, so it doesn't come between the subject and the main verb.
It also puts more emphasis on the "I" instead of the "no one". In some cases it could be clearer to have a positive subject rather than a negative subject.