They were all afraid because of Tara's DID. She knew it, they knew it, and yet none of them really understood.
Kate was afraid that she would end up with the same problems as her mother. It's why she was bold, honest, and willing to do anything at least once. It's why she hated when anyone made even the slightest connection between them. It's why she tried desperately to find her own way of getting through life. It's why she was always running as far away from home as she could. She couldn't stand the idea of being split into pieces, but she also admired her mother. She found herself surprisingly in awe of her mother's strength, vulnerability, and compassion. She found hope through her mother's struggles, which helped her continue to push through her own.
Marshall was afraid of hurting anyone. He saw how easily the littlest thing triggered his mom, and how her alters then destroyed so much around them. It's why he struggled with his sexuality, because he didn't want to disappoint anyone either way. It's why he picked up cooking as a hobby, so he could be helpful. It's why he couldn't break up with the one girl he actually dated or be honest with his first boyfriend. He was terrible at confrontation, at trying for what he wanted, and standing up for his beliefs. His family was slowly coaxing him out of his shell, but every so often he could feel how easy it would be to slip back in.
Charmaine was afraid that her mentally ill sister was coping better. It's why she felt compelled to stay as close to her as possible, both physically and emotionally. It's why she valued her sister's opinion more than she liked to admit. It's why she convinced herself that she should value things in life that she truly didn't care that much about. She couldn't deal with all of the trauma in her past by disappearing into atlers like her sister, but she didn't think her way of dealing was any better. At least, she couldn't believe she was doing better if her sister had the supportive husband and two great kids while she had tried justifying taking the baby of the man she couldn't admit she loved away from him while marrying a guy that she had always thought of as a pushover and a bit of a jerk.
Max was afraid of where the love really lied. He felt a strong bond with all of the alters, and sometimes wondered if they loved him more than Tara did. It's why he stepped lightly with her, letting her decide whether or not to take her meds and what boundaries he would keep. It's why he didn't fight all that hard to get the alters to leave. It's why he stuck around despite the fact that the alters often cheated on him. He was afraid that if he ever tried to rebel against the system, Tara would take the alters and leave him. His greatest fear was that he would miss the alters more.
Tara was afraid she would disappear. That her alters would take over, and she would be stuck inside watching, or worse just gone. She was afraid no one would notice her missing, or that they wouldn't care. She was afraid that the alters would destroy all of the people she loved most. She was also afraid that her alters would disappear, leaving her all alone. She was afraid that she would miss them, and how they always seemed to know what she needed no matter how much she argued to the contrary. But her greatest fear, was that her family would find out that she didn't much care how it turned out, she just wanted it to happen already.
They were all afraid. But they found comfort in the denial that each one of them offered the others. After all, that's what family is for.