Technically, what's happening is that the MIL is selling her residence and giving all of her children a portion of the proceeds, to do with as they will.

When you say multifamily, what size are you thinking?

All shapes, sizes, ages, etc. 

Unless you are hiring a mega builder who gets insane discount on labor and material, most of your medium size or small size builders should have access to similar cost labor and material. 

This zip code has a high population density with a substantial portion of the population in age groups likely to rent (22 to 34 years and under 18 years), suggesting a steady demand for rental properties.

My understanding is: most sellers will pay their commission, but what if they don’t or they only pay a portion of it?

As a CA RE Broker myself thats how I did all my loans and virtually all of them were fractionlized given the size of your average CA loan..

Bonus depreciation allows you to deduct a significant portion of the cost of qualifying property in the year it's placed in service.

@Elizabeth Seiferth if you DM me suburb and scope size I will connect you with one or two people we have worked with.  

Consider Tiffin.Des Moines offers even more options because of its size and spread.