Saya menonton film itu dua kali, tetapi adik saya belum pernah menontonnya.

In Indonesian, most transitive verbs (verbs that take a direct object) are formed with the meN- prefix. The base verb tonton (“watch”) becomes menonton when you actively watch something. Specifically, the prefix meN- assimilates to men- before a t-initial root, so tonton → menonton (“to watch”).

itu is a demonstrative pronoun meaning “that”. By saying film itu, you specify “that film” (perhaps one you’ve mentioned or seen together), in contrast to film ini (“this film”) or just film (unspecified).

In Indonesian the measure word kali (“times”) follows the numeral. So you always say dua kali (“two times”) rather than kali dua. The typical order in the clause is:

1. Subject (Saya)
2. Verb (menonton)
3. Object (film itu)
4. Adverbial phrase (dua kali)

• tetapi and tapi both mean “but.”
• tetapi is more formal; tapi is more colloquial.
• In standard writing, you put a comma before either one when joining two independent clauses:
  Saya menonton film itu dua kali, tetapi adik saya belum pernah menontonnya.
  Omitting the comma is common in speech or informal text, but in formal writing it’s recommended.

• tidak pernah = “never” (it has never happened, no instances at all).
• belum pernah = “not yet ever” (it hasn’t happened up to now, but still possible in the future).
  In your sentence, adik saya belum pernah menontonnya means “my sibling hasn’t watched it yet.”

The suffix -nya here is a third-person object pronoun meaning “it.” Instead of repeating film itu, you attach -nya to the verb:
menonton + -nya → menontonnya (“watch it”).

In Indonesian possessive structures, the noun comes first, followed by the possessor. So:
adik (younger sibling) + saya (my) → adik saya (“my younger sibling”).
Reversing the order would be ungrammatical.

Yes. sudah marks completion (“already”). Including it is optional here because dua kali already implies the action happened. Adding sudah emphasizes that you have already watched it twice.