M.S.
Last night I slept 8 hours straight, for the first time in almost 3 years. My kids are 13 months apart and my daughter is now 10 months old. She was a horrendous sleeper until recently, but she just started sleeping through the night (12 hours without waking) consistently. This means that 5 nights of 7, she'll sleep well. Every baby is different - my son didn't sleep through the night until he was a year old, no matter what we did. My first recommendation is the book by Weissbluth, Healthy Sleep Habits, Happy Child. He recommends an early bedtime (for a 9 month old, probably between 6 and 7 p.m.) as key to getting quality sleep.
Until your youngest is sleeping through the night, take him out of his brother's bedroom. If you don't have an extra bedroom, put a crib or pack n play in your room and have your 9 mo old sleep there until he gets it together. We did this with our daughter. Actually, I slept on the couch while she was in our room, because then she had to be soothed by Daddy. As an experiment at 8 months old, I stopped nursing before bed and gave a bottle of formula instead. Our daughter went from a 2 hour stretch of sleep to 8 hour stretches of sleep.
I found Ferber to not work for us, but neither did "no-cry" options. Every time we went in, our son would freak out MORE when we left, and that would lead to vomiting. We did all out cry-it-out with him - which was our absolute LAST attempt (before hiring someone like you suggest), and it worked in 3 days. The first night he cried for 1 hour 6 minutes. The Weissbluth book says it's ok for them to get so upset they vomit, as long as they are safe and cared for. He even said it's OK to wait until your child is asleep to go in and clean up the vomit.
I have seen a lot of harsh judgment on this forum about crying-it-out - and everyone has to find out for themselves what works for their own families. It can be hard to stomach, but it worked for us, with both our kids. And as a result, we're finally sleeping. I wish this for your family too!!