I would like to apply for a visitor visa for the following people:
My father
My mother
My grandmother
My aunt
My question is that can I launch one application for all of them or should I launch one application for my parents and another for my aunt and grandma?
Yes you can put them in single application, since they are family members.
However you need to get "Use of a Family Member Representative for Online Applications" i.e. IMM5713E signed by all family members to appoint (lets say your dad) as the representative for the visa application.
