My boyfriends almost ten year old has been begging me for a sibling since I met him when he was…

My boyfriends almost ten year old has been begging me for a sibling since I met him when he was five. He has two siblings at his mom's that he adores.. is it rude of me to want him to have as much time with mine like he has with his others? Me and his mom get along very well but when it comes to certain things you just don't know how she will take it. I think she will be excited for us.. I've spent time with all her kids and treat and love them all like they are mine and she likes that.. I hope it goes both ways ya know?