It seems that "sister" in ukraine language is very similar to the one in English. May there be a connection, I wonder?
Yes, those words are related. Most European languages (except Basque, Estonian, Finnish, Hungarian and Maltese) come from the same ancestor, Proto-Indo-European language, and sometimes the common origin is still visible.
Thanks a lot. I heard indo-european language family before. What does Proto mean? Is it another family group or sub-family group?
Proto-Indo-European is the common ancestor of all Indo-European languages. Proto comes from Greek πρῶτος meaning ‘first, earliest’ (I looked it up in Wiktionary, I don’t know any Greek myself), so it’s like ‘the first/earliest Indo-European language’.