At eight, this isn't a safety issue any longer -- she is unlikely to wander into traffic or get lost in a crowd. She may be holding hands for her own sense of security, or to demonstrate affection, or for whatever reason she wants to.
If that's fine with the person she wants to hold hands with, then it does not need to be considered an issue. When it is a source of conflict, and somebody else (older sister, parent, etc.) doesn't want to hold her hand, then it's time to have a conversation about boundaries.
She doesn't have the right to force somebody else into physical contact, whether that's holding hands, hugging, tickling, or kissing. She's definitely old enough to understand the notion of boundaries and personal space. She has the right to turn down unwanted physical contact, and she also has the responsibility to respect other people's right to the same.
I'd also suggest that the person who doesn't want to hold hands be a bit more accurate (and considerate) when responding, though. Saying "you're too old for that" is rather disingenuous: is the other person really hoping to hold her hand, but feels obligated to refuse because of the age boundary? I can see why that comes off as sort of unfair -- would the sister also call her childish for wanting a hug? In any case, simply saying "I don't really want to" is more accurate and honest, and reminds your younger daughter that the other person has a right to refuse.