I would upgrade to a new 200 AMP panel and get the tallest one in that you can fit. This panel upgrade should be done first and if possible the new panel can be placed as close to the existing one. We have used old panels as large splice boxes before in the past.
You will need city permission and permits to set this up and often there is a long wait time to get the power company down to cut the main power.
Check if your even allowed to help your brother in law and insure he gets the proper permits as a home owner.
Simple drawings will need to be made to pull your permits so I would start with those.