@caringcarer thanks that is useful advice and what we will do. Particularly the bit about asking the agents to price it to sell.
@ForensicAccountant FIL has never lived in the house, he owns another house that he lives in. He has moved since our house was purchased but he has always owned a house separately to our house.
@FinallyHere - no that is not what I mean, when I posted that I was actually searching on line to see what kind of return he would have got on the money if it had been invested in an ISA which is what he would have done with it if he hadn't bought into the house. That would likely still be less but if we can't afford to buy him out at market value it might be an option that would allow him to help SIL out without requiring DH to sell his long term home.
@SW18Life FIL acknowledges that this is not what was originally planned and that it could be challenging for us.
@Notthetoothfairy correct, the house is held as joint tenants so would revert to DH with no IHT and no chance of SIL making a claim on it.
The original plan was that SIL would get a greater share of the estate to compensate her. Her and her partner were already on the property ladder when FIL helped DH out, they had traded up a couple of times and owned a very nice cottage in a desirable area.
FIL says he has some more money, some of which he doesn't want to use because it is in a fixed term ISA (I assume that he also wants to leave himself a cushion of savings which is sensible). He doesn't think he can afford to buy SIL a house outright even with the proceeds from our house, he thinks she will need to get a small mortgage, but whether she will be able to get one I don't know.
If when FIL and MIL both pass there is nothing left due to care home fees or just due to their living expenses then that is fair enough, we aren't the type to count on an inheritance and have been encouraging them to enjoy themselves for years. However I can completely understand that it could be very damaging to the relationship between DH and SIL if there was a situation where there is nothing left and we have debt but SIL inherits a big chunk of equity in her own house.
@Seracursoren a financial advisor might be a good idea. I'm not convinced that FIL will be able to do what he would like to, and I am concerned that his investment could be at risk if SIL stays with her partner.
Thanks for the recommendation of the broker I will have a look. I want to ask a broker whether we would be better having the mortgage in just my name or jointly given that DH is self employed. Had we been planning to move he probably wouldn't have given up employment, or would have got himself a job to help with the costs and getting a mortgage but I don't think we have time for him to get a job and for it to make a difference.