My wife won't let me help my sister

I pay most household expenses, because my wife's salary is just over VND5 million ($205). I work hard to take care of my wife and children. However, these days, my wife and I have been arguing about an issue that we can't seem to agree on. Over time, I've found my wife to be greedy and selfish.

My brother-in-law worked as a driver. But two years ago he died in an accident. Before his death, he was hospitalized, which cost more than VND200 million. Since then, my family has helped my sister and I've paid her children's school fees. My parents moved from the countryside to live with her in the city to help her take care of the kids. They take the children to school every morning.

My sister is a factory worker. Even when she works overtime, her salary is only VND7 million a month. Her life is difficult. She has to both raise children and pay interest on loans. I'm very concerned for her. My parents are not able to help her financially. She's losing weight and suffering the anxiety that debts bring.

She was planning to borrow money from me a few hours ago to pay off a bank loan because the interest on the loan is nearly 14% year and she is unable to pay it. I currently make VND50 million per month. My wife and I have VND700 million in savings. When we borrow, the bank offers us 7% interest.

I asked my wife if we could help pay my sister's VND400 million debt to a local bank. I told her my sister had already agreed to sign a notarized contract that she'd pay back the loan at the same interest rate the bank charges.

My sister owes VND400 million to the bank on her mortgage. And she had to borrow more to pay her late husband's hospital bills before he died.

I feel sorry for my sister. But I also feel uneasy because my life won't let me help her. My wife thinks she won't be able to pay the debt and would never give us the house that she and my parents are now living in.

What should I do in this situation? How can I help my sister while also keeping my wife satisfied?