That's rude to guilttrip like that. There are plenty of women who do it for fashion or comfort reasons and they look amazing like Amber Rose, Alek Weck or Grace Jones as examples over several decennia, just as Natalie Portman, Kristen Stewart and Charlize Theron. While heads are often shaved in support and hair is sometimes seen as an offering in some religions (like Hinduism), it doesn't mean that it's exclusively related to suffering. It just is something your mom has defined it as.

What's also important is that not every bald head is because of cancer. Some women get alopecia, some women have religious reasons, some just like to switch things up; they aren't all sitting around resenting hairy people. And if it really is something cancer related: depending on how long your sisters hair is, she can donate the locks to be made into a wig so one bald woman can have hair again.

Your sister has bodily autonomy and should feel free to exercise that. It sounds like your mother is very worried about how people view her and that should be more important than how happy her kid is with her look. That's a selfish attitude.