During the trial, without a lawyer, can the family members sit on the plaintiff's bench with the plaintiff to reply?

No, you can only go through the entrustment formalities.

civil code

Article 58 A party or legal representative may entrust one or two persons as agents ad litem.

The following persons may be entrusted as agents ad litem:

(1) Lawyers and grassroots legal service workers;

(2) Close relatives or staff members of the parties concerned;

(three) citizens recommended by the community, units and relevant social groups where the parties are located.

Article 59 To entrust another person to represent the lawsuit, a power of attorney signed or sealed by the client must be submitted to the people's court.

The power of attorney must specify the entrusted matters and authority. An agent ad litem must have the special authorization of the client, and can admit, waive or change the claim, make a settlement, file a counterclaim or appeal on his behalf.