It is quite likely that the premature delivery was related to her abuse of pills. Not only this, there is also a risk that the newborn may show intoxication or withdrawal effects related to the drug abused by the mother. I feel your sister-in-law might have tried to stop abusing these pills when she got pregnant, but was unable to tolerate the withdrawal symptoms
and resumed the habit. If she is willing, advise her to see a doctor and take professional help for deaddiction. There is a possibility that the second child may also suffer from adverse effects.