You may say 'imouto.'
older sister- Aneyounger sister- Imouto
姉 Ane
onee-sama
Shinnai naru shimai
Imouto is for your REAL younger sister and imoutosan for another person's younger sister.
Onee- Older sister (most common, though there is one more)
The term 'little sister' could be translated as 'imouto', to say 'my little sister' you could say 'watashi no imouto'. But remember, if you are going to refer to someone else's little sister, you must say 'imouto-san', 'san' being the honorific, which are a key element of the Japanese language and etiquette.
"Sister" in English means sorella in Italian.
"Your sister" in English is tua sorella in Italian.
"Twin sister' in English is gemella in Italian.
"Not with my sister!" in English is Non con mia sorella!in Italian.
It is 'ane,' written: 姉