I have a character who was named after her grandmother. Halfway through the story, the grandmother suddenly shows up. Neither goes by their full name. One is Lanni. The other is Lonna. I'm concerned the names are too close and will be confusing. Any suggestions besides a different nickname based on their full name? That's not going to be doable.

Could call the grandmother "Granny" or something similar.

I had thought of that, but I don't know if that would work for her sister or the people they're traveling with that Granny just met or the one who knew her in the past and has reason to hate her.

Grandma Lan Lan and Grandma Lonna

Probably far too late, but a mixture of things might work. The two girls might refer to their grandmother as Nan or something similar, and the grandmother may still use her pet names for the girls. The people they've just met probably won't want to call them Lanni and Lonna because they're so similar and may use Lanni and Grandma Lon (or Lonna and Grandma Lan if it's that way round) to avoid confusion. The person from Grandma's past might have a spiteful, derogatory name for her which he/she would delight in using.

Not too late. Still working on it. Calling Lonna Lon might work. The group is notorious for shortening people's names. Thanks

I guess the granddaughter could be called "Little (Name)". It's kinda how it went with my mom and grandmom, who shared the same name.

My uncle and my cousin did similar until my cousin was taller than my uncle. Then it was young and old. But the granddaughter is very short and sensitive about it. Young and old are questionable because they're both immortal and over a thousand years old.

Speaking as someone who shared a name with both my grandmother and a great aunt from a different side of the family, I know all too well how confusing this can be. We solved it by using my middle name, which nobody else in the family had. Perhaps that might work, especially as it has no associations with elder/younger, junior/senior, big/little, etc. that might not quite apply or cause the named-for character to be extra sensitive about any tags or diminutives.