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

Set a time to do this. It is an important issue and deserves a “scheduled” timing. You can each come up with a list of what needs to be addressed. For example, are there any problems (e.g., are they interfering or intruding into your marriage/family?) to be noted? Which are the aspects of your lives that needed boundaries to be set (child discipline issues? Disclosure of information? Visitation frequencies? Etc) Discuss your lists and agree on which and what boundaries are reasonable. Once done, both of you will have a clear understanding on the boundaries you have agreed on. Communicate them to your in-laws if required (e.g., the ones on disciplining child). It will also be good to review and amend the list as time progresses.