"I have one sister."
Translation:У мене є одна сестра.
You can say this, if you want to stress the amount of something. For example, if someone asks you "How many sisters and brothers do you have?" you can answer "У мене одна сестра і два брати." Generally, if you just want to state the fact that you have something, you use the phrase "У мене є ...," which corresponds to the English "I have a ...."
This alternative is now accepted too.