Yes she absolutely is old enough to pack her own luggage but if they both want to do it then whats the problem? Maybe its something they enjoy doing together, a bonding experience or sharing the excitement of going on a holiday. I dont see the harm in it,and unless one of them has told you or given the impression that its a problem then I would stay out of it as its not harming anyone.
I would assume your sister is also old enough to say no if she wants / needs to.
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