The word "同志" can mean 'comrade', 'fellow' or 'pal' and as a slang 'gay' or 'homosexual' ... Idk why the author used that word (he could have used 'gege' for old brother [ 哥哥 ] and 'mèimei' for young sister [ 妹妹 ] ) but it probably is because they are close siblings like friends(?), so I think the proper translation should be brother and sister instead of comrade to fit the context in English. Thanks for the chapter