If it’s one sister then say: “It’s my sister’s thing”. Is it we’re multiple sisters then you would say : “ It’s my sisters’ thing”.

Anastasia has a good answer for your last question. For the first question, yes, you're right that [ 's ] can be possessive, or it can mean either "is" or "has." All of these uses are extremely common. Here are some examples with the meaning of "has": "It's been three days since I saw him." (It has been three days since I saw him.) "He's been calling me all week." (He has been calling me all week.) "She's been to Spain twice." (She has been to Spain twice.) For your second question, this isn't really a usage of a possessive [ s ]. "Its" is a complete word, meaning "which belongs to it." We don't put apostrophes in possessive pronoun forms in English, include "my," "her," "his," "their," etc. You should just think of "its" as one of these possessive pronouns. Even thought "its" happens to end in an [ s ], the [ s ] is not what makes it possessive. "Its" is possessive in exactly the same way as "her" or "our." The [ s ] is not a separate, added element that needs an apostrophe.

Not sure if someone else already covered this. ITS is the only time a possessive doesn't use the apostrophe. I suppose it's because they let the contraction for IT IS (IT'S) have the apostrophe.

It's my sister's thing.