There is no verb for "to have" in Hindi, therefore the meaning must be created through various other constructions.
In the case of "having" a relative, this is how you need to say it. This is a different sense of "having" then you would say if, for example, you "have" a key (i.e. something you can hold in your hand). In the latter case, you say /mere pās/ (kind of like "near me"). In the former case (this example), it's like one is saying, "of-me, two sisters, are". Remember, /merī/ is just the grammatically proper way of saying /maiṅ kī/ (of me).
maiṅ kī SISTER = sister of me -- although this is wrong because it must be / merī SISTER/. My point is that what is really being said is "sister-of-me."
तेरी translates to ‘your’ so तेरी दो बहनें हैं would (literally) be translated as ‘your two sisters are’ or ‘your two sisters exist’. However, that’s not how you phrase it in English. You would not say ‘your two sisters exist’ but rather ‘you have two sisters’
While तेरी (and equivalently तेरा and तेरे) means ‘your’, तुम means you (or you could say तु, which is more informal or आप, which is more formal)
You are a sister would therefore use तुम instead of तेरी.
You are a sister: तुम एक बहन हो
ter-X ("your") is a possessive pronoun. Possessive pronouns work like adjectives. They must "match" the thing they refer to.
Take the adjective, baṛ-X ("big"):
baṛā kuttā - "big dog". [kuttā] is masculine, so baṛ-X must have [ā] ending.
baṛī billī - "big cat". [billī] is feminine, so baṛ-X must have [ī] ending.
baṛe kutte - "big dogs". [kutte] is masculine PLURAL, so baṛ-X must have [e] ending.
Do the same with [ter-X].
terā kuttā - your dog terī billī - your cat tere kutte - your dogs