My sister is older than me and she just had a doughter and she told me that when she didn't know she tried some drugs and got adicted... she finally stopped but will the baybe be OK

Only time will tell. It is pretty hard to tell how the baby will be effected. I have seen babies of addicted moms that were not effected and then some that have. It depends on a lot of things.
Also the extent of any harm is hard to tell. I believe that part of the reason kids are having problems where they need drugs such as Ritalin is because of the addictions of one of the parents. So I believe the effects can vary as well as the extent.

You didn't say what sort of drug she was taking as this would indicate what type of problems the baby could have.