I don't want to discourage her but My niece had the same problem..but although all the creams and medecines, her lips remained black..

Maybe your sister is allergic to the chemicals found in lipsticks. Try some herb based lipsticks. The darkness of her lips will fade out with time. Try Aloe Vera natural lipstick.

Tell your sister to abstain from lipstick for the time being. Give her lips a complete rest. Ask her to place cucumber over her lips to rejuvenate the lips. With time the skin of the lips will be worn out and be replaced by new ones which will be of her natural color that is how it was before. After regaining her natural color tell your sis to use a good brand lipstick. Try Avon lipsticks.

My friend have dark lips too, she was told there isn't any cure for it unless she want to go for lips embroidery for a lighter colour. Most people say it's the residue of the lips colour we used that cause the change in pigment, but some guys don't use lippies but still have dark lips too!

I bet she must be using lipstick of different brand. Tell her to stick to only one product. The different composition in the different lipsticks has caused such a repercussion. Lemon juice is a good natural bleach. Try that

Ask your sister to apply some lemon juice on her lips but not for too long; after 2 minutes wipe the juice off her lips. Continue this process for about 2-3 weeks I’m sure her lips will regain its natural color.

Tell your sister to compress her lips with rose water and stop using any lipstick. Her lips are either too sensitive or allergic to the brand that she is using. Ask her to use only lip balm till her lips return back to normal. And use only natural lipstick onwards.

After brushing your teeth, brush your lips lightly with some Vaseline on your lips.
Next, thing would be to apply lip balm with SPF 15 before going to sleep.
Drink a lot of water everyday.
While doing makeup, cover your lips with concealer then apply lipstick. For better results apply lip liner too.

I’m very sorry for your sister.
Your sister must have been experimenting lots of lipstick on her lips. This is not a good habit. Tell her to use only one brand and which is adaptable to her lips.

Hi, stop using the lipstick that you used to put on your lips, i advise you to go to a pharmacy and buy a moisturing lipcare.