I had the same thing happen with my now 6 year old, he slept through the night by 10 weeks, then started waking up in the middle of the night. I also had a 5 year old at the time. I didn't want to let him cry it out and wake up my 5 year old. The pediatrician's advice was it would be better to have 3 or 4 nights of bad sleep than 3-4 years of it. Meaning, if I continued to let him come into bed with us, he may make a habit of it. At 7 months, it's much easier to break a habit of him waking up than at 12-24 months. It worked. I let him cry it out. From what I remember, he cried ~10 minutes the 1st night and it was less each night until he slept through the night again. It probably took ~ 4 nights. Again, since your daughter has already slept through the night, she has already established a good sleep habit. On the other hand, my oldest never slept on his own through the night until he was close to 2 because I would always bring him in bed with us. I tried letting cry it out at 18 months, it didn't work with him (I think partly because of his age, partly his personality). So, if you know that everything is ok, try the crying it out for a few nights and see what happens. Good Luck!