If you get on with them then move in, worse case if you start to niggle each other then move into rented. It’s a good opportunity to cut down expenses and save some money, I’d grab it with both hands. Agree on some ground rules first though and either pay a nominal rent or put some away each month in case it ever becomes an issue. When you move out you could always treat the mother in law to a holiday or something out of the money she saved you.
As for the brother in law, wealth advisor? What the hell is that? Sounds like an amateur. He can guarantee you 12k worst case, 15k best, on what level of investment? Does he stay up all night locked in his bedroom playing internet poker? Seriously, don’t mix money and family.