My only sister died in 2006.
In actual fact, having re read your question, having brothers or sisters is not relevant.
If a parent leaves a gift children on the child dies leaving their own children, then under section 33 of the Wills act, they get the share which their deceased parent would have got.
However as this is not your grandmother who now died and which your mother predeceased, and there is no blood relationship, then the gift lapses and falls to the residue in your mother-in-law’s estate.
I’m sorry if this answer is unfavourable.
Can I clarify anything?