Objective: The stress of having a child with cancer can impact the quality of relationships within the family. The current study describes the longitudinal trajectory of marital, parent–child, and sibling conflict beginning around the time of diagnosis through the first year of treatment. We examined the average level of marital, parent–child, and sibling conflict at each monthly time point in the first year of treatment; the proportion of families that fall into the distressed range of marital, parent–child, and sibling conflict at each time point; the typical trajectory of conflict during the first year of treatment and whether there are differences in trajectories across families. Method: A total of 160 families of children newly diagnosed with cancer (Mage = 5.6 years; range = 2–18 years) participated in a short-term prospective longitudinal study. Primary caregivers provided monthly reports of marital, parent–child, and sibling conflict. Results: Using multilevel modeling (MLM), most families showed stability in quality of family relationships, although considerable between-family variability was observed. For married couples, 25–36% of couples were in the distressed range at one time point over the first year of treatment. For married couples, more distress occurred at earlier months, particularly month 3. For parent–child and sibling dyads, the most difficult time periods were during later months. Conclusion: Implications for development of interventions that target at-risk family relationships are discussed. Identifying processes that predict between-family variability in trajectories of family relationships is an important next step, particularly for the marital relationship.