Could you give up on a loved one?

I have a sister that lives off the grid and she doesn't keep in touch with any of her siblings and their are 6 of us. But she would usually call me at least every 2 years. But I haven't heard from her in 3 years now. Only me and my brother are the only ones that care. I try talking to my sisters about it because I am concerned, and they just take a nonchalant attitude about it saying "You know how she is" and "she doesn't want to be found". I just don't get how someone could don't care about their own blood like that.