why did he say the brother and sister having the same husband thing??? im still confused

oop, sorry man followed you by mistake.

Same sex couples in their world still have husband and wife roles, for "reasons" Ian is the wife and Michele is the husband, so Ian was warning Katya that if she got too close to Michele (he actually doesn't want them to conspire against him) there might be rumors that a brother and a sister (Ian and Katya) are sharing the same husband (Michele)