"I have a sister."
Translation:Ho una sorella.
I don't think you can differentiate the subtle difference between those 2 sentences in Italian - they both translate to "Ho una sorella". You can highlight that there's only one sister by saying something like "I only have one sister"
And to be honest I don't think you will need to differentiate the two sentences. Most of the time, "I have a sister" is correctly understood to mean you have one sister. The times when it doesn't mean that you have just one sister, there will always be additional context (in both English or Italian) eg. The sentence "I have a sister who writes books" - it's quite possible that you have more sisters.
And remember, sometimes Italian has shades of meaning that get lost in an English translation (prime example: single/plural and informal/polite forms of "you" - tu, voi, Lei, Loro).