K.J.
My daughter was a great sleeper right from the start, my two sons were the opposite. They also shared rooms so I know the problem you have. My son ended up not sleeping through the night til 18 mo because I didn't want him to wake up my daughter. If I could do it over I think I would have had her sleep with us for a week or so until we broke him of it. With my one year old who did not share a room as an infant and also had sleep issues, we tried this: Let him cry, after 3 min went in, said ok go to sleep patted his back a sec and left. After 5 min, went back in did the same thing, after 10 min or so went back in and so on until he fell asleep. I did not pick him up as that prolonged the process. After a couple weeks of that he was falling asleep fine at naptimes and sleeping through the night. He was 10 mo and before that would only go to sleep in a stroller or in my grandmothers lap and then we transferred him to his crib.