My 3.5 year old wants to be by us at night too, in fact we moved her twin bed into our room and put it next to our king bed so it is like one giant bed. We all love it, she sleeps great next to us and she is content. She knows that when she is ready she has a nice big girl room to sleep in, but for now she is comfortable where she is at (plus we are all sleeping great).
I say don't worry about it! Just tell her to try not to wake you guys up when she crawls into bed and eventually she will outgrow the need and be fine in her room with her sister. There is nothing wrong with out little one's wanting to be near us at night, I think it's natural and it helps them feel safe and loved.