I'd probably use 20% combined considering the low rents and probable tenant class.What about water/sewer, snow removal, lawn care, and admin/professional fees?

@Andrew Carlson I think you are combining two separate things: 1. tax advantaged retirement accounts and 2. investing in publicly traded securities (stocks, bonds, mutual funds, ETFs, etc). 

I underwrite to 15% combined.

They all cost some combination of time, money, and personal effort. 

You can probably pull back the repairs and CapEx to 15% combined, @James Bennett.

They have a lot of unique loan products including a 30 year commercial note.

You not only need to be trustworthy and have a solid reputation but you need to show competeance in your field.That being said you can meet lenders and investors at REI meetups and groups, auctions, talking to realtors, title companies, property management combines, craigslist calling for rent adds, other investors, search real estate transactions in your area and look for multiple cash closings by one individual, entity or address of either. 

This is a unique area where there is an advantage to all size rentals.

Yeah...that's not going to work...and will likely make the situation worse.If your parents want to keep the property and keep things amiable, they should offer to buy him out (or some combination of your parents, you and/or your siblings should offer to buy him out) at FMV. 

Maybe, if it was adjacent to a commercial zone and all the residential neighbors had to have a coffee shop and would support you till the end and the city wanted to bring in that specific type of business and the lot was so unique that it had to be there.