If you're naming the child after your family members, it only makes sense to me to spell and pronounce it as that family member does. However, I do believe variations of the name are ok (for instance, your grandmother was just
Ellie but you want to name your daughter
Eleanor after her). But
Marissa /
Marisa seem too close to be used as a variation to honor the other, if that makes any sense. For the record, I prefer
Marissa and prn. mah-RISS-ah. To me,
Marisa is mah-REE-sah.