Conundrum regarding new will for mother in law

[welwynrv]

Right little long winded but here goes, my mother in law fell & broke her leg 3 years ago, it did not heal well and she ended up in hospital for nearly a year having the leg rebroken, skin grafts etc and had about a dozen operations. Fast forward to last year she had permanent limited mobility as the repair didn’t go well & she was starting to show signs of short term memory issues. *Me & my husband had been discussing selling our house & moving away from Hertfordshire as I was no longer commuting in London & asked her if she’d be interested in coming with us as my husband didn’t want to be hours away from her. She said she would but only if my husbands sister would come as well as she is bipolar & has an 8 year old daughter. *My sister in law was very keen but couldn’t contribute as she was on benefits & lived in council accommodation. It was first discussed us buying a house & then mother-in-law buying a place for herself & my sister in law but we quickly realised there would be stamp duty implications so it was decided that we would sell our property our mother in law would sell her property and we’d both put £250k into the pot to buy a property large enough for all the family pay all the associated costs buy new furniture etc, my sister in law contributed nothing. Our plan was to split the household bills 50/50 with mother in law leaving sister in law *to contribute when she could afford to. We sold our house first so mother-in-law took out a bridging loan for £150k for us to complete the purchase of the new house pay all the expenses etc. We moved in December & the sale of my mother in laws completed on 21/02 she then transferred her outstanding amount to us. *All the household bills repairs etc since the move have come out the pot. My sister in law has given me £100 towards the household bills, my mother in law has given nothing so far as she has been unwell & unable to get to the bank, she doesn’t do phone banking etc. *Last week adter feeling unwell for a month she was taken into hospital as it was suspected she had pneumonia, it turns out she has terminal lung cancer and has at most 6 months to live. *My mother-in-law was always going to split her estate 50/50 between my husband & his sister and the house is only in mine & my husbands name, we were going to get something draw up to say mother in law could stay in the house forever etc but now she has a terminal diagnosis and needs to update her will. If mother in law had stayed where she was my sister in law would have just got 50% of the estate after expenses, trying not to be selfish we don’t want to be forced to sell the house to pay off my sister in law as we’d probably loose money and we’d both still need to buy properties. my sister in law is happy living with us but can’t really afford to contribute much. Looking at it basically we owe her half of what mother in law contributed to the pot but do we then have to take into account moving costs household expenses etc or does she has a 25% claim on the house and we fix it at today’s value so she gets her share in the future or will she be entitled to 25% of the value if we don’t sell for another 20 years and prices crash, can we offset any of the ongoing household expenses against what’s she’ll be due. I’m getting a solicitor in to do a new will for her but need to understand how we protect our investment*