"This girl is my sister."
Translation:Эта девочка - моя сестра.
Девочка is used for a very young, under-age girl (usually under 16). What in English you would call "little girl"; a female child. However, you can call your girlfriend девочка - just like in English couples often call each other "baby" though clearly they are not babies...
Девушка is a grown-up girl, a young woman - like молодая женщина, though the latter would be considered a small step above just a girl.
Мой is used with masculine nouns -Он МОЙ брат(He is my brother)
Моя is used with feminine nouns-Она МОЯ сестра(She is my sister)
(As far as I know)Меня is a word used in the expression "У меня есть" (I have),is used to indicate that it is YOU the one who has something,I don't know if it can be used alone or combined with other words,I'm a learner just like you,hope this helps !