varkarrus's Links
(non-canon)
Amvira can and would be a "big sis" / even an adopted parent for Nix. After the events of her story, with all her friends getting families and settling down, Amvira would start thinking she'd want a family of her own. Being aro/ace, she wouldn't be planning on getting married, so adopting is exactly what she'd do (assuming this is before story's return to space).
That said, I feel like Amvira would have a hard time getting Story to open up to her. She may be somewhat empathic, but she's blunt and has almost no experience raising a kid (she probably thought raising an older kid would be easier!). She might even get really frustrated with him at times! But, since this is after the events of her story she wouldn't be an ass to him, or get angry. If anything, she'd just be a bit mad at herself.
That said, once the supernatural stuff starts happening in Story's life, Amvira would be right there, ready to fight alongside him and defend him. She's practically omnipotent by this point, so she wouldn't let anything harm him!
(non-canon) Story would definitely be... put off, to say the least, at first by Amvira. He has trouble getting along with people as it is, and they're complete and total opposites. She's tall, loud, lazy, and overall definitely a bit intimidating to him. However, I think if given plenty of time and patience he would eventually get over the fact that his new mom is an antagonistic demon, and perhaps start looking for more of the good traits in her. He would definitely appreciate the protection and loyalty she would provide, and might even try his best to be a voice of reason and a comfort to her. If she were there at the time he was taken to Pluto I think he would appreciate it greatly, since in the devastation of the planet she would be a source of familiarity and protection.