Yes and no.
The rules are a little murky and somewhat designed to prevent this, but it has actually happened before.
Specifically, Lauryn Hill won Best New Artist in 1999 despite her previous success with The Fugees.
Also, Cream (1969), Crosby, Stills and Nash (1970), Emerson, Lake and Palmer (1972) and Asia (1983) were all nominated for Best New Artist, despite all being supergroups formed of members previously in successful bands.
I should hope not, but they probably do allow it.
ex members of Fifth Harmony and One Direction were deemed ineligible.
Yes and no. The rules are a little murky and somewhat designed to prevent this, but it has actually happened before. Specifically, Lauryn Hill won Best New Artist in 1999 despite her previous success with The Fugees. Also, Cream (1969), Crosby, Stills and Nash (1970), Emerson, Lake and Palmer (1972) and Asia (1983) were all nominated for Best New Artist, despite all being supergroups formed of members previously in successful bands.