First, your own sibling relationships help shape your expectations for how your children might get along. My brother and I got along great growing up. We played together when we were little and hung out fairly often through college. I expect my girls to get along. When they do play and hang out, I count that as it should be. My husband and his siblings didn’t get along so well. His older brother and he fought often and never felt close. His younger sister and he bickered often. When he sees the girls getting along, he is still surprised. He thinks it’s just short of miraculous they enjoy each others’ company.
Second, how you speak to and about your grown siblings models to your children how to speak to and about siblings. Read that again if you need to. When your children are within earshot, speak about your siblings in the nicest way possible. It’s great if it’s honest, and it’s okay if it’s a stretch, or just avoid saying negative things so openly. I speak very openly about growing up with my brother, how much fun we had on family vacations and how it was great to be at the same high school and college for a year. My husband speaks nicely about his sister and avoids speaking much about his brother as it’s still rocky.
Third, you may side more often with one or the other based on birth order or other related variables. I was the youngest in my family, and I find myself occasionally siding with my youngest Claire because her perspective makes sense to me. The goal is to recognize the tendency and be sure it doesn’t become a pattern.