Being in a family that is nice to each other is not exclusive or in any way more likely for Mormons, or really any religion.

If anything it could be easily argued that it would likely trend away from religious households, but without solid data I think it's better to assume it's most likely to just be the same likelihood regardless of religious or spiritual beliefs.

However, in the case of Mormons, they actively train their followers to effectively disavow even their own family if they choose to move away from the religion.