The BlackOps challenges just have to be different for each heir. You can pick them up through skills, work, or some special challenges.
Sorry, but you can't add families. I can't remember who posted about getting new people into town, but basically, if you send children to school, have them keep meeting new friends, and if someone has a job, have that person keep meeting new co-workers. Also, if you read the paper every day, you'll get the names of new families who have moved in.
Yes, you do have to be friends with them.