The plaintiff can not serve the defendant but it is okay if a family member that is not part of the action serves the complaint. The best bet is to use a registered process server, they do not charge that much and they know what they are doing.
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If they are 18 years and not a party to the action, then yes. If you expect there to be a dispute about service, then hire a private process server or have a friend do this for a few bucks, because if you have a close relative serve it, the other side will try to say that they have an interest, if service is disputed. Be sure that you complete the proof of service and have it signed by the server.