What have you done, young man? If it's 20k PL from bank, I assume they've checked your details even before approval is given? What puts you in such desperate situation right now? Did you lose your income?

The quote below most reflect the reply I'd have given you.
1. Never go behind your wife's back. You start this little thing then it grows bigger and when you realize what happened, you've messed up the relationship.

2. Never even borrow money from family members if possible. Worse if elderly. Everything comes with a price and don't even let me begin talking about we are a family, why need to return kinda attirude. Yes, you're desperate now you can promise heaven and earth to return the money, but once you get the money, human pride tends to come back. It's just the nature of human.

3. As a man, deal with integrity in the things you've messed up. Never take the easy way out, especially at the expense of a relationship honesty and retirement fund of an elderly, regardless of what relationship you both have. I don't touch my parents money I've stated to them clearly it's for them to enjoy life it's fruits of their labor so they should enjoy it. So don't dare to even think you deserve borrowing from your mil.

Your case is fortunate as you are dealing with banks which offer ok ok interest rate as compared to along. I hope you can find a solution to it soon.