Learn Turkish: İkiz kardeşlerim var ama mizaçları tamamen zıt. - I have twin siblings, but their temperaments are completely opposite.

You can remove ama (but) and add ise after the topic of the second clause: İkiz kardeşlerim var, mizaçları ise tamamen zıt. You can also attach it as a suffix: mizaçlarıysa. This emphasizes the contrast, translating roughly to 'I have twin siblings; as for their temperaments, they are completely opposite.'

Why use İkiz kardeşlerim var instead of just İkizlerim var?

İkizlerim var usually implies that you are the parent of twins ('I have twins'). If you want to say you have siblings who are twins, or that you are part of a set of twins, you must clarify it with kardeş (sibling), resulting in İkiz kardeşlerim var.

The root word for temperament is mizaç. Why doesn't the ç change to c here?

Consonant mutation (where ç becomes c) generally happens when a suffix starting with a vowel is added. Here, the plural possessive suffix -ları starts with a consonant, so mizaç remains unchanged. If you were talking about one sibling's temperament using the singular possessive -ı, the word would mutate to mizacı.

Is there a verb missing at the end of tamamen zıt?