My sister owns 17 SFH as well as an expensive personal residence in a different nearby town.

I would love to work side-by-side with someone with experience so I can gain experience of my own as I build my confidence in doing my own STR for an investor who may not have as much time running a STR. 

Pretty much all that have been mentioned here between the two vehicles.a lot of the other items come into play based on your own specific financial situation.so for what you have provided that is all 

Owning a property with the sister is about impossible as she is not good with money or business minded.

The MIL has owned the property for 4 years and there is no mortgage (purchased in cash by her grandson; I raise this to you in case there are capital gains implications for MIL and DIL when the latter sells). 

Since each lender can make up their own rules with HELOCs, there's no way to know what a particular lender will or won’t allow unless you ask them.

I own a business that builds modern, prefab tiny homes (or cabins depending on how you look at it), and I am looking for a market where I could set up a few of these to use as model homes, available as a STR.

We just need a little bit of cashflow each month to make it work and we are wondering if anyone else pays themselves to GC their own deals. 

I own 2 rental properties, the first one is paid in cash and the second one i have a mortgage on it which i pay 464 in total including tax and insurance, trying to see how much i would qualify using only my rental income while i look for a job once moving there in ilhaving a hard time qualify for a loan since they cant use my job income that i have here in cali, been told that I need to move there first and work at least 1 month and then apply or I need a job offer letter. 

FYI, insurance make consider each unit a separate claim with its own deductible.