When my partner and I started talking about becoming parents, there was an underlying assumption that we would have one kid and be done. But I can pinpoint the moment when I first thought about having a second child. Our son was 15 months old and his favorite book was one about baby animals and we read it to him so many times he had most of it memorized. We sat cuddling on the couch in the living room and my partner and I would name an animal and he would gleefully respond with the corresponding offspring — eaglets, piglets and kits. I said, "Otters have…" and he excitedly yelled with an adorable lisp, "Whelps!" I was so overwhelmed with love for our little family that I remember thinking to myself, "I want to do this one more time."

Not too long after that, my mother was hospitalized in Kansas City and we drove down to see her. It was the beginning of a slow and steady decline due to multiple health issues and my sister and I sat together, taking in all of the information and making plans. Later, when we drove back to Minneapolis, I admitted to my partner that I didn't know what I would do without my sister. Dealing with my mother's health issues and my complicated relationship with her would have been so much harder without my sister. We have a shared history and could support one another in ways that no one else would have imagined. I told my partner that I wanted our son to have a sibling for those reasons and more. She wasn't as certain about having another child because she liked our life the way it was and I said, "If we have a second child, he'll always have someone who understands his past."

She countered by saying that siblings aren't always close. I knew that, of course. I have two brothers, too — one with whom I am not as close as I am with my sister and another I haven't spoken to in years. There are no guarantees.

We did eventually have a second child, three and a half years after our son was born. Our kids have had moments of unbelievable tenderness but have also had some epic fights. They have played and laughed together and they have pinched each other and pulled hair. They know each other's secrets and vulnerabilities and are not above using them to their advantage. Once, after a particularly emotional and dramatic fight over cake decorating, I sat them down and told them that we were not going to live forever and the kind of relationship they have will ultimately be up to them. It was all love and light for a few days and then back to the usual mix.

A couple of days ago, I was working and they were yelling so loudly that I could hear them even though I was upstairs and wearing headphones. I went downstairs ready to intervene and they were sitting at the dining room table laughing and talking. It turns out they are as loud and rowdy when they are getting along as when they are fighting. But I also realized in that moment that the number of arguments has been dwindling and they laugh more than they fight.

Today, they are flying to Pittsburgh to visit their grandparents. My son told me that I needed to make sure that my daughter understands he is in charge. Then my daughter made it clear that she was in charge of herself. He held up his hands in surrender. "Fine." I told them both they are in it together and are supposed to look after each other. They assumed I was talking about their trip and I was but I was also talking about so much more.