We always advocate exclusive breastfeeding in the first 6 months. We also know it can be difficult to breast feed with a flat nipple.
But as it is now, your sister is already tensed as a result of the difficulty to breastfeed.
Let her know that emotion and state of mind can affect milk production. How often she eats and relax matters too. She should check all these and make them balanced. Definitely she will produce more milk for the baby. She should take plenty fluids like water, pap e.t.c and can also put baby to breast often, the suckling reflex will induce milk production.