Is it legal for plaintiff to use the plaintiff's family members to serve the summons to the defendants??
Probably not because they are an interested party. Use someone over 18 who is not related to you.
Workers' Compensation Lawyer
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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Legal Information is Not Legal Advice
My answer provides information about the law based on the limited information provided in the questions asked and is not intended to be legal advice. The law differs in each jurisdiction and may be interpreted or applied differently depending on the location or situation. I highly recommend that you consult with an attorney to discuss the specific details of your situation so you can get legal advice tailored to your specific circumstances. The information in my answer is for educational and information purposes only, and is not legal advice or legal opinions. The answer provided to the question asked does not constitute a lawyer-client relationship.
Lawsuit / Dispute Attorney
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.