Yes there was imbreeding but I went to a christian school in my youth so this is the explanation they gave us.
During Adam and Eve's time on Earth the Earth was like a hyperbaric chamber (very few of these chambers exist, but basically you heal faster in these, you can run farther, etc.) so the earth was this huge hyperbaric chamber. You could run faster for longer, yada yada because the earth was brand new. That means that our genetic structure was perfect. So Adam and Eve's genetic structure was perfect, which meant hypothetically if they had a sister and brother, they could have sex with them and have children without any deformities. Then, when Adam and Eve lied to God about eating the fruit, sin came into the world and with that our genetic structure started to change and become imperfect. However, it didn't get to the way it is today just like that, it took time. So by the time it came to Noah's time, the Earth was still somewhat like a hyperbaric chamber so everyone was healthy, and our genetic structure was better, well better than it is today. So that's why they could have kids with their brothers/sisters and cousins, etc without the children being deformed.
Whether or not this argument is true, I'd have to do more research on. But I'm basically regurgitating what I was taught.