Cry it out.

I'm thi king of doing the cry it out sleep training but going in every 4-5mins if she gets too upset and reassure her, if she wake in the night should also do the same? Letting her cry it out until she resettles herself, I also have a 2yr old and i dont want to disturb her sleep as she is a great sleeper. I dont want to have two crying children in the middle of the night and have to get them both back to sleep. I can totally do the cry it out at bedtime that would be fine but when she wakes in the night I do anything she wants just to make her go back to sleep as quickly as possible. She is 9 months old. Shes never slept properly all her little life and I need to start getting my sleep back. She doesnt nap properly either I do it by awake windows but they seem to change everyday I try to get her down every 3hrs from the time she last woke, and sometimes she will sleep 1hr + and sometimes its only 20-40mins. It's not consistent although i try to be as consistent as I can as well as sorting out my other daughter too. She needs to start getting enough day and night sleep as she just isnt a happy baby. Thanks for your advice in advance