My suggestion is: call your IP without telling them who you are (withhold your number too), explain the situation to them, and ask them if they would claim the money. If it were me running your IVa I woulnd't claim it because it;s not your money, but I can't speak for other IPs.
If your IP says Yes, they would claim the money, then perhaps you should explain to your sister-in-law what the situation is and get her to put the money in a trust fund for your benefit once ethe IVA ends.
You might also like to ask them whether it's OK to borrow from your family to replace the car/do household maintenance on the basis that the loan is non-repayable until the IVA ends. I think they should certainly agree to this.
Philip Beck -
www.freeivaadvice.co.uk
Licensed Insolvency Practitioner and IVA specialist