Here are a few stories:

We were going out with his twin and waited for him to show at one spot. He was running at a field nearby before that. My husband and I couldn't find where he was running and then twin's cellphone broke down. My husband was panicking a bit coss he thought something happened to his brother. After 20mins his brother showed and my husband said he was worried sick about him. I was surprised at first bit then I heard that years ago twin fainted somewhere and realized that was the thing my husband was afraid of. I then realized they bond is like of parents and their children. 

The other time my sister, her friend and me just finished running a half-marathon and resting nearby. My husband ran also but didn't sat with us for long. He waited his twin to show, who was running a whole marathon. Of course, the brother said he would finish in 3 hrs, but he came after about 4,5 hrs. My husband was mad and worried the whole time for him.

The last one; me and my husband had a relationship crisis at some point, and it was my bad, and I already figured I almost made a huge mistake. The one person who was the maddest about that situation was his twin, who then had a conversation with me and gave me a reality-check, saying he will do anything to stop that situation from ever happening. (meaning if he sees that I'm messaging someone he will go to my parents house and tell on me!)..uh...anyways.. I am so gratefull for the conversation.