She does not have money for her expenses and is borrowing at a high rate of interest which is becoming impossible the idea is to sell my cousins rental property take a boot to cover her expenses and the rest ($750,000) do a 1031x.  

While interest rates are high compared to the recent past, they are in line with the historical norms.

Unless you come across an opportunity that has much better cashflow, I don't see a reason to get rid of a property that has decent cashflow with a low interest rate in the current environment.

Higher interest rates continue to make it more difficult, especially for an investor that is looking to house hack, cash flow, or DSCR loans on the commercial side are a nightmare. 

Builders are buying down the interest rates to 5% making payments work for cash flow.

Would it be better to do a sub-to and take over their existing mortgage and rate OR get a PML?

They are a smaller bank giving out quite a bit of loans for new construction cabins in the area as STR investing is on the rise. 

Sometimes in states like OH or PA, a lender might require you to buy it out, which increases your rate. 

You may find one out performs the other by a lot especially with interest rates (and HML rates can kill what might look like a good deal otherwise).I would be happy to take a look at the specific property you are interested in . . . send me a message with the address rather than posting it here. 

Typically rates are better and you can go up to 90% LTV from your primary house.