I was in a similar situation.
My siblings were basing the valuation on the top figures from agents. They also had an out of date impression of the house. I lived in it, (was renting it since relative went into care). The boiler was very dodgy, the electrics were in a poor state, there was a small leak in the kitchen extension roof...).
I did a spreadsheet using agents' valuations
Added in agents' costs, higher solicitors' costs with thorough buyers who'd ask more questions ( - I wouldn't need surveys, proof of XXX etc), likely time to get a buyer, maintenance costs while waiting, costs of clearing it out, (all of which wouldn't be paid if I bought it on the spot).
I got two "we buy any house" type offers. They were low but offered what I was offering (ie instant cash) and so I said that was a fairer comparison.
I then said if I didn't get an answer by DATE I'd move out and leave the property unrented and they could drive down every week and mow the grass and check the post.
They did the maths and realised selling to me would save them a huge amount of money.
Interestingly one of them was in a similar situation when their own MIL died. They still haven't sold her house one year on and it's costing a fortune.