If a will is witnessed by someone who is a spouse of one of the beneficiaries or a civil partner of one of the beneficiaries, then the bequest to the beneficiary is invalid.
However it must be a partner who has gone through a legal civil partnership, not just partners who live together.
I think it unlikely that a civil partnership because they are not usually undertaken by male and female, just two people of the same sex. However, it is possible.
If the witness is the civil partner in a civil legal partnership, you might not want to say anything! That way, the will will just be largely invalid.
If it is your sister-in-law’s intention to split the estate proceeds 50-50, then there is no reason why the will should not have said that.
If the sister tries to keep everything for herself, your husband has an excellent challenge either for undue influence, the inheritance (Provision for Family and Dependents) Act, or lack of mental capacity at the time was written plus of course, depending on the status of the partnership, being incorrectly witnessed.
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