Should I have more bridesmaids?

I want my sister to be my maid of honor, mi sis in law a bridesmaid, my little cousin (7 yrs old) to be my flower girl and my friends daughter (4 yrs old) to be my ring bearer. All of them will be girls since I don't know any boys. There will be 90 guests. Can the bridesmaid walk down the aisle alone? There will be no groomsmen. How does it usually work?