If you are a teacher, never make interviewing a family member a part of an assignment.
“Interview an adult” or “Interview a person who fits this description” is probably okay, but many people do not have families, and many people who do have terrible families. Forcing someone to spend extended time with a family member may be contravening their usual strategies for avoiding violence. If they live on their own, you may be trying to force an escaped abuse victim to recontact their abusers.
Not everyone is able to approach you for an alternative — abuse often comes with fear in social situations and problems interacting with people. They also don’t know that you are going to be willing to give you an alternative; you could potentially refuse, be abusive to them yourself, report the abuse (possibly because you’re legally mandated to) and cause serious problems in their lives that way, cause a runaway minor to end up in juvenile detention or just returned to abusive guardians, or even contact their parents and tell them the student is telling lives about them.
In addition to abuse, I’ve seen examples of things like “interview two/three family members.” An only child raised by a single parent may not HAVE two family members to interview. Disclosing your family structure and/or parent’s family abandonment should not be necessary for your grades.
It’s fairly simple to come up with an alternative. If you want them to talk to adults: “Interview [number] of adults about their experiences… Parents, teachers, librarians, and family friends are examples of people who you could interview.” If you want them to talk to people close to them: “Interview [number] of people whom you have close relationships with about… Family, good friends, and romantic partners* are examples of people who you could interview.” (That said, ‘people you’re close to’ runs the risk of being exclusionary to people who have trouble forming relationships so be careful with this one.)
This is true for any age of student, kindergarten through university, though I suspect it comes up slightly less often in university.
*Obviously this one’s only going to be middle or high school and up, but yes, your students may have romantic partners and may be very close to them.