
Originally Posted by
imdeng
You can of course right another letter just for the grandmother and keep it in reserve, just in case. Assuming you are all in clear with your documentation, your parents and grandparent should not have any issues, especially since there are going together. It is unlikely that they will give visitor visa to some in the party and not other - and your parents case is clear cut - so would say your grandmother's is as well.
I am only passing secondary information BTW. I only have experience of visitor visa for parents and not grandparent.