It's been a while since I've read the book, but I can deduce that since his sisters were both younger than him, maybe his parents were actually better parents to his sisters than to him, and his mother knew it, and his father didn't care enough to change anything before he died.
As for Gudrun, I remember reading how sexually liberating the 1920s were for women and since Britain got the head start on civil rights for women, Gudrun represents what was ahead that maybe Lawrence wanted to show and explore.