I need to buy a four million dollar house. How would I go about doing this?

I have a very special situation, and an interesting backstory.

My parents are selling their home as they retired and are planning to travel around the world with their time.

Both my sister and I want the house, and my parents will not pick between the two of us. They do have some sympathy towards my plight, however, so they are waiting until I say that I definitely cannot buy the house before selling it to my sister.

My sister is a successful lawyer, and her husband works on Wall Street. She wants the house because she knows she can easily flip it and sell it for more as my parents are selling it for way below the actual value. She lives in New York -- she has no need for a house on the west coast.

I, however, am a social worker, and am unmarried with two cats (I guess I m a cat lady then?). I believe I can pull together maybe a little less than 2 million in down payment, and my yearly salary is very variable. This house is the source of many happy childhood memories, and as I regularly temporarily foster children, the extra rooms will be very useful. I can also start a miniature hobby farm for the children to work on in order to hopefully ease the trauma of the first few days. Because my parents know that I am desperate to keep the house, they are willing to hold on to it for me. I think they also may be willing to drop the price down to 3.5 million for me.

Is there any way for me to afford this house? I would be very saddened if my sister sold it out of greed.
Added (1). I clearly am unable to afford it. It's what I thought before, but I guess I was just hoping for something to jump out of the woodwork.

Thank you to all who answered.

My parents know that my sister is planning on reselling -- that is why they are holding out for me to buy it. As for needing this house, yes I certainly don't NEED it, I just really hoped to be able to keep it. I've given up a lot to become a social worker, and I do not regret it one bit, not even at the sake of this house.

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