Newsflash: Am I being paranoid?

I want to preface this by saying I am a 19 years old worrying about my parents making a stupid decision. We currently live in New York City, and we are fortunate enough to own two rental properties that are both 3 family. One is worth 1,000,000 and the other one is 1,600,000 dollars. In terms of mortgage, each has 400,000 left with a 5% mortgage. The problem is they want to buy a 2.1 million dollar single family house with 100% leverage. I know you are confused on how that is possible. The way it is possible is they took out a HELOC for 650k on the 1.6M home, and then they will use that as a downpayment on the 2.1 million dollar home. Another important part to consider in their regular job. My dad is a taxi driver, and owns a taxi medallion and total income is about 150k from his own job, but from the two properties its about another 150k or so. I am concerned that this is a dumb idea, and will result in default since mortgage payment in total is about 15k a month. I need some thoughts on ideas on this situation.