My 2 yr old niece has alternating esotropic issues. her pediatric eye doc wants an mri. her granddad has strabismus. how likely is this is a mass?

MRI would help: Less likely depending on how suddenly she developed the esotropia. Normal MRI would be reassuring to rule out a mass

Extremely unlikely: Alternating esotropia is a coordination problem with the eyes in which one or the other eye takes up fixation but they do not work together. Mass lesions if they affect eye motion will usually cause a paralysis so that one eye moves poorly compared to the other. Mass lesions also are likely to cause papilledema which I assume was not seen by your pediatric ophthalmologist.