I don't know if I understood well, but I'll add an example of what I understood and you'll tell me :
define hero = Character("[player_name]")
define sis = Character("Lola")
define mom = Character("Ariel")
define friend = Character("Elsa")
label start:
hero "My name is :"
$ player_name = ("What is your name ?")
$ player_name = povname.strip()
friend "Hey [player_name] !"
sis "Hey bro !"
mom "Hey son !"
Hope this looks like what you intend to get.
Now if you want to consider custom names, like "Brother" instead of "Bro" for example, you could just :
default from_sis = ""
default from_mom = ""
label custom_names:
$ from_sis = ("How does your sister call you ?")
$ from_sis = povname.strip()
$ from_mom = ("Same for your mother")
$ from_mom = povname.strip()
sis "Hey [from_sis]"
mom "Hey [from_mom]"
Remember that the Character class is only here for the renpy.say function, and the string variable you'll store as a name is just used for writing it in textblocks. Any other string variable can be used for anything. Hope this was your issue and that I fixed it, if not, I'll gladly help you again :)