I would say that it wasn't that good usually but they still loved each other (no homo) given they were brothers. Zach even admitted that he and Nikolas didn't get along well and that he "bullied him" because he thought he was weird. They often fought a lot, which was one of the main reasons that the police were called to the Cruz household often (ie. they were called once because Zach and Nikolas were fighting over the computer). Based on interviews with Zach it seems like they both got along with each other when they were young kids (they used to go to the community pool together) but started to not get along when they got older. Additional evidence for this comes from Zach's lack of care/ignoring blatant signs of mental illness displayed by Cruz. Like Zach mentioned that he saw Nikolas cutting his arms and blasting music and all he didn't say anything or tell anyone since "he was always weird like that" or something along those lines. Ultimately though he was still his brother and did love him or care for him (Nikolas) and vice versa given with how they acted when Zach visited Cruz in jail (both were crying and hugging). So I guess their relationship could be described as not good but not horrible.