"Buying into" existing rentals owned by family

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First post. Regular lurker. Mortgage lender with 14 years of experience, but coming to the experts humbly and looking for your thoughtful responses.

My sister and brother-in-law own a couple of rentals that are performing well (I did their loans.) Both were purchased in the last year.

They may be selling their primary residence and moving. They likely cannot get enough out of that sale to make a substantial enough of a down payment to their liking. Enter my proposal...

What if I buy into their existing rentals? In other words, I give them cash to become a partner in their rentals. Maybe we even refinance to get me on the loan if they wanted that. But bottom line - I give them $$ and I now own 50% (or whatever we agree on) of their rentals. They get the extra down payment funds for their next home (without having to do any cash-out refi's of any homes) and all is well.

I realize there is paperwork, agreements, and understanding involved. But assuming all of those things look good, this seems like a nice way to get them the cash they want, AND give me some ownership into their rentals (we would form a LLC to clearly define ownership percentages.)

So - what are the pros and cons? There is an explicit trust between us, so that would not be an issue.

Thanks in advance for helping me think through this.