We are at the last stages of Probate after my mothers death Intestate last March. Mum sold the family home in December 2015 having been pressured quite heavily by my sister to do so. Whilst I knew the house was up for sale, sis didn't tell me a sale had been agreed & I went over to find it empty with mum moved out 
(I hadn't been around for a few weeks as my daughter was in hospital for 3 weeks on the south coast). My relationship with sister has been bad since that time. Mum moved to a rental whilst she was finding a permanent move but then passed away in March a day after her 89th birthday.
My sister previously had a building company & over several years she would do stuff in mums home like put in kitchen cupboards that had come from somewhere they were refurbing or if she was paying staff for a full day on a contract but they weren't needed on a job they would sometimes do a bit of work at mums - this was over a number of years, a bit here & there. Mum also paid her for major things like a new Boiler & getting electrics done, these were at normal costs.
We were all grateful that she did this though Sister constantly let us know how amazing she was for doing these things for mum. Mum gave money to sister on a very ad hoc basis over the years & told me she was upset that sis had told her friends how mum owed her for all the stuff she'd done.
There are 4 of us, but with a brother who was more of a hindrance than help & another sister abroad, all support for mum over the years was done between myself & sister.
After the sale of the house (within days) mum told me she had been taken to the bank by sister & her husband and made to sign a document & pay £165,000 to my sister. The document is brief and states the money is in payment for the works done on the house, mum signed it & it was witnessed by a friend of BIL.
We were never rich & the house happened to be in an area that has 'come up' in London, so for the first time in her life mum had a large amount in her bank.
Forward to Probate (sorry this is so long!), we are awaiting HMRC to let us know if they are viewing the 165K as a 'Gift' which then will incur a further 40% tax bill - which sister has not agreed she will pay, so this means that any further possible tax will be shared between all 4 siblings!
Finally - my question is whether there is any way we can either 'force' my sister to take the hit on any further tax payable or claim that she has already had a large percentage of the estate & therefore this should be reflected in the final settlement of the Estate (the Estate was v simple with no other property/shares/etc.)? The initial tax has already been paid to HMRC.
Sorry this has been so long. I'd be really grateful for any advice....