Siblings of children with chronic disorders are highly underrepresented in research. Living with a child with a chronic disorder affects the whole family, including siblings. Research has revealed negative impact and several risk factors for family and siblings. However, two risk factors have been emphasized as especially important, namely poor family communication and the lack of disorder knowledge. This thesis is based on an open trial of an intervention for siblings and their parents. The research questions are investigating if 1) the quality of family communication, and disorder knowledge in siblings will increase from pre-to post intervention, and 2) quality of communication and level of knowledge are related to each other. Method. This thesis is based on a one group pre-post design intervention study. The Parent-Child Communication Scale was used to measure siblings-rated family communication, and the Sibling Knowledge Interview was used to measure disorder knowledge in siblings, both at pre-and post intervention. The sample consist of 101 siblings and their parents. The age of siblings ranged from 8 to 16 years (M = 11.5, SD = 2.06) and 54.7 % of siblings were girls. Results. The results showed that quality of communication increased significantly from pre-to post intervention, but only for sibling-rated communication with the mother. The level of sibling disorder knowledge also increased significantly from pre- to post intervention. Increase in the quality of communication was significantly related to increase in disorder knowledge, but only for communication with the mother, not the father. Conclusions. These findings support the use of this intervention for siblings of children with chronic disorders. Furthermore, the findings represent an important step in exploring who will benefit more from the intervention. The findings show that both communication and knowledge improves. Thus, siblings with poorer family communication and lower disorder knowledge could be in greater need of the intervention than siblings with better family communication and more disorder knowledge.