My sister is 30 weeks pregnant and thinks her partner hasn't stopped doing drugs. She wants to know if she is having unprotected sex, will his sperm affect the baby in anyway?

No, but she has much worst issues, being with a drug addict

In her tummy?

Depends on what else the drug user is involved in. Who knows what?
Sex during pregnancy: What's OK, what's not - MayoClinic.com (http://www.mayoclinic.com/health/sex-during-pregnancy/HO00140/NSECTIONGROUP=2)

Anything that the pregnant woman ingests will effect her unborn child. Anything that goes into her bloodstream passes through the fetus.

His sperm cannot harm her baby, unless he has a serious transmutable disease that can infect the mother's vaginal canal.

I also wonder about the title, inside "tummy" exactly how old are you ?