Privet.

From my limited experience I learned that the Insturmental case is used in structures such as:
1. Студент пишет ручкой
2. я разговаривал с тобой
3. водка с лимоном
4. я пошёл сестрой
5. я пошёл со своей сестрой

My question is why in some cases we use "с" and in other cases we just leave it? Is there any rule?