Is this a realistic story idea?

I'm writing a book about a girl who's parents split up (before the story takes place), she is 13, her younger sister is 9 and her older sister is 15. Her mum meets a new man and they move in and get married, he has 4 kids, a 7 year old girl, a 11 year old boy, a 13 year old girl and a 15/16 year old boy. Do you think the ages are too close to be realistic and do you think his older son should be 15 or 16? Also in what situation would a mother leave her husband with 4 children, even if she got access to them. Don't worry about the storyline and whether it's sounds like your sort of book, this is just the logistics of it.