How do we figure out the boundaries that we’d like to set with our in-laws?

I think in-laws should be mature enough to respect boundaries, After all they would have gone through a similar situation. However if you feel intrusion coming your way, there's no harm in having a respectful discussion with them making them understand why the need to keep boundaries. I feel anything explained with SBA (Situation, Benefit, Analysis) will help them understand , accept and appreciate you better.