So as you've posted in Money Matters rather than Legal Issues, I'll offer an uninformed perspective! But it might be worth cross-posting to get a view from the solicitors who post on MN.
My initial reaction is that the entire value of the car should be deducted from her half of the estate. The car is part of her inheritance - the fact FIL hadn't paid for it outright is beside the point (although she should reflect on why an elderly man was taking out car finance to provide her with a car she could presumably have saved up for herself).
So let's say the car was bought for15K, rest of the estate worth 85K to make nice round numbers. 50K each but SIL's 50K is 15 of car + 35K cash, 5 of which she uses to pay off the remaining finance.
Strictly speaking I think what's meant to happen is FIL's estate pays off his debts first, so now there's 95K in the pot to be split, which is 85K cash and 10K of car. She gets 10K of car + 37.5K of cash, DH gets 47.5K of cash.
However, it sounds as if she's arguing she should get the car AND half of the remaining estate AND an extra 5K to pay off the finance? Sod that.
Gifts (not that the car necessarily was a gift, depending on who was the actual owner) need to be considered when valuing the estate, you can read about it here at MoneySavingExpert. I think it would be worth consulting a solicitor to get the specifics.
My questions are:
- Who is the owner of the car? Is it SIL (so the car was a gift) or FIL, with SIL as the registered keeper.
- Did FIL benefit from the car before his death, i.e. was it bought for SIL because she did all his shopping / took him to medical appointments etc?