Look it's simple,Mario's older than Luigi by at least a year,probably 3. In yoshis Island,they -had- to make them twins for the plot. It just woudn't work out if luigi wasn't born yet!!! Then they wanted to go back to the good ol' Big Bro Lil' Bro theme so no matter how screwed up it made everything nintendo did it as it is the Mario way.
Thank you for reading this little monolouge,come again soon!