Stuck, it's only misinformed or lazy doctors who claim that it's the cause -- when the sufferer is a mature, reasonable person who insists that he/she does not bite or lick his/her lips obsessively. I find it so offensive that doctors would say that the patient is lying about this.
It may be something in the environment around you (in your house, for example), or something in the foods you and your sister eat, or bath/skin products you both use, or a fungal infection that both of you picked up (that is very easy to do when you share bathrooms and kitchens with people), that is causing both of you to have the lip inflammation/exfoliation.