My boyfriends sister! HELP!!!

Ok let me start off with I love her! She's adorable, really she is. The problem I have is that she's almost 12 and still all over her brother. It's fine it's just she's almost as big as him and she still wants to cuddle with him and sit on his lap, like all the time. She needs a lot of attention with I understand she's a kid and she's the only one in the house still. Her brother (my boyfriend) is 23 and lives on his own but his mom still treats him like a kid and still asks him to lay with his sister and do everything with her.