I actually thought the custody subplot was decent, if mostly because all of the actors made it believable and the backstory of Mike having lost a little brother he failed to watch over to William and the massive amount of guilt he has over it.
The evil aunt was way less realistic because of her character to begin with, and actress portrayed her way more hammily compared to the mostly more realistic portrayal of the protagonists and even some of her goons by their actors, to the point where she felt more like a Saturday morning cartoon or sitcom villain, but she was so over the top and unapologetic about her selfish goals that she felt right at home in the same universe where Fazbear Entertainment exists.
It ain't the most original storyline, but for what it's worth, the actors gave it their all to make it work.