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Hello and welcome to Just Answer.
Sorry that poor Reba is having this problem.
Since you can't look at her safely, the best I can do is give you some general suggestions. I will tell you that it is important that you have your veterinarian take a look at her to get a definitive diagnosis and treatment plan.
Some of the more common reasons for a female to be licking her vulva are bacterial infection of the skin of the vulva/around the vulva, yeast infection of the folds of skin around the vulva, urinary tract infection, allergic skin disease and urinary incontinence (causing her to lick the dribbling urine to keep herself clean).
Less common causes of this behavior are foreign bodies in the vagina (in California this is usually the awn of a foxtail plant), maggot infestation (yuck!!) and tumors of the vagina.
Most of the things listed in the common causes section are fairly simple to treat with either topical or oral medications, but you have to be able to see what is going on to make a diagnosis.
I hope this helps and certainly don't get hurt trying to look!