Can Sole Executor Change Terms of Will?

[Davros]

As sole executor if my mother's will, can my DSis alter the amounts to beneficiaries and/or add beneficiaries? My other sister milked my mum dry when she was alive and we would like her to get a bit less

Of course not.

No, of course she can't! She has to make sure she carries out the will of her mother.

no!!

Boo hoo, I thought so.

Beneficiaries can alter terms to their disadvantage (eg DSsis decline bequest so that it goes to DGC instead) but the executor cannot make arbitrary changes, it goes against the whole ethos of the role.

No not at all and its not her decision to make. As un fair as it seems to you and your DSIS you have no idea what her actual relationship with your DM was.

I know of situation where one sibling has accused the other of the same but it wasn't true at all it was literally sour grapes.

We have hard evidence of the milking. Ideally we would start by suggesting that two of the beneficiaries choose to have less but this is unlikely

If the milking took the form of loans, could the executor in administering the estate not insist that they are repaid or set off against her share of the estate?

Yes, debts have to be repaid before anyone receives any money. Of course that means they will receive a percentage of their repayments back, but that's better than nothing. Proving it's a loan might be difficult, though.

I don't think we could prove that the £1000s were loans

The thing is, it was your mum's money, not yours.
How she chose to spend it in her lifetime, and how she divided it on her death was up to her, not you or any of your sisters.
It might seem unfair (and probably is) but she could equally have spent the lot on cats.
Or do you feel your mum was handing over her money under false pretences?