赤ん坊の妹 /a kan bou no i mou to/.
You may say 'imouto.'
zan zumi
Just say it.
chokoreito no akachan
ko gitsune
petite soeur
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.
"neko no onesan" which basically means "cat's big sister"
妹妹 (mei mei) meaning younger sister
Niece = female Nephew = male
You may say 'imouto.'
Imōto no futago no imōto