I don't know her age but she could be at that age where the influence of others is big. Everyone goes through a point where they do something stupid for others, so it could be her friends, or she has just decided she's more interested in the vally girl scene. [personally I don't see who would but some do enjoy that lifestyle] so you could just view it as a phase, keep an eye on her, but don't panic.
In regards to magick effecting her, yes, someone could theoretically cast a spell to make her act different, but its like love spells, you need to overpower her free will and it wouldn't be perminant. Your parents dabbling in 'black magic' two decades ago really shouldn't effect her but some lingering energy could effect the atmosphere of the home. In which case a cleansing and protection should be done.
You can also cast over another even if you're far away [its just a bit more tricky] since she's your sister you know where she lives and have her photo. You can cast a spell using the photo, or direct the energy onto her where she lives. Just visualize a protective bubble around her and it should work. Keep in mind casting over another regardless of the reasons should first be discussed with the person and permission granted for the spell.