hello there ,
Thinning of the lips is caused poor periodontal support and bone loss due to loss of teeth.
Loss of the teeth leads to resorption of the supporting alveolar or socket bone , resorption in turn leads to flattening of the bone .
as the bone height decreases the musculature of the supporting lips becomes loose and the lips appears thin.
i would suggest you to visit your dentist
and get yourself examined.
clinical evaluation is required in your case and accordingly suitable treatment can be planned.
prosthetic appliance can also be given in your case,but this can be done only after prosthodontists evaluation.
hope it helps.