there has been a thread about this before, with lots of good advice, so do a search. I live overseas and so know lots of people who have babies at home and then fly back here (7 hours from Europe). The norm is around 6 weeks, after your 6 weeks check, although some mums have been known to return when baby is only 4 weeks. I think doctors usually do recommend that you don't fly with a very small baby because their immune system is still immature and they will be exposed to such a large crowd at the airport and on the plane.
I seem to remember that 8 weeks was recommended because the baby has had first set of jabs.
The link between SIDS and long haul flights caused panic for a while, but it was later disproved, and there is no SIDS advice now not to fly.
I flew back here when ds was 8 weeks. To be honest I couldn't have sat down for 7 hours until he was about 6 or 7 weeks, let alone done the whole airport/long flight thing.
The first few days here were hell as he was awake until 2am, crying, because of jet lag. It took him over a week to settle on to the new time, so think about being guests in someone's house with a crying baby all night, and you and dh feeling stressed and exhausted because you aren't sure what to do to help him switch time zones. We have been back to Europe when he was 9 months old, and he switched time much, much more easily.
We lived with my parents (who are fab)for the first 8 weeks of ds life, until we came back here, and I would definitely think twice about going to stay with in-laws (even if they are fab) until you and baby have found your feet.
and my ds was a lovely happy baby who sleep through at 5 weeks, so it would be much harder with a less easy baby (not suggesting yours won't be an angel of course )