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.

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.

*also available in ShareCG, DAZ, and CGBytes flavours.