Hi there,
okay so Felicia grew up with a full family until just before she started at Hogwarts. Her family were mostly killed in a pretty horrific 'car crash' (She has yet to realise it wasn't the crash that killed her parents or most of her siblings).
She was raised by her mom and dad until she went to Hogwarts, her little sister left with an aunt she stayed with on the holidays so she could see Willow. Once Willow joins her at Hogwarts they stay for the holidays.
In terms of blood family her little sister was the one who had the most impact on her life, especially when they are both handed over to first Umbridge then Bellatrix and the Malfoys. Felicia takes a great deal of trauma to try and save her sister, although what really breaks her is hearing her sister screaming for her when she's tortured.
Willow is the one sibling she allows herself to remember, but she had three others at one point. None survived the crash so she clings to Willow quite a bit.
She's very close to Willow, of course, and she grows incredibly close to McGonagall very quickly when she comes 'home' after the Wizarding War. She also welcomes Hermione to the family when Willow falls into that relationship, remembering Hermione's kindness even when they were locked up together in Bellatrix's basement of tortures.
Felicia was always the eldest sister so she's never really been able to relax fully, although once she loses three of her siblings she and Willow start to act almost like they were always the only kids. Both Felicia's dad and mother thought of her almost as a third parent and she still tries to keep her place as a surrogate mother for Willow although Willow also guards her now, the two bound even tighter by their pain during the war.