i had 30+ btc then when btc reached 19k $ still i didn't sold & i am not selling now either
Well at one stage you will have to sell, if you want to pay back your sister at the $300 per coin price. I reckon you do not like your sisters husband a lot, so why not rather place some of this money into a fixed savings account and then pay her a monthly interest? Obviously you can give her a higher percentage of the returns on the coins, because $300 per coin sounds a bit unfair.
Just do a 50/50% split to pay for your recovery skills and deposit that into a fixed savings account. This way the Bank will be paying back your share from interest.