1a and 2a are wrong; totally unnatural.
1b and 2b are grammatical, but are strange in the context. Another way of saying this is:
"Isobel, John's sister, had heard the joke before." This is far more natural; it just identifies who Isobel is, for those who don't know
Isobel, but who do know John. For this reason, 2b is better than 1b.
Thanks for taking my other advice. It doesn't always happen.
Student or Learner