Sometimes what baby "wants" isn't what baby "needs" it's up to M. to decide when it's a "need" or a "want". Of course you never just let the baby cry for hours on end, but some times you have to use your judgement. If baby's been feed, cuddled, changed and laid down then it's sleep time. I let my 2 month old cry a bit sometimes, sometimes I need to do some things before I can go to him. He's not happy, but he's not in any danger. The CIO method isn't as cruel as it may sound. With my daughter I never just let her scream the farthest I was ever from her was on the other side of her door. Even then I was speaking to her, letting her know I was there but that it wasn't time to be awake. I would not do that with my son right now, he's too young, when he is older I will. When will he be old enough? For me, for us, we shall have to see, so far he's a pretty good sleeper.
CIO isn't for everyone, it's hard listening to your baby cry, specially as a breast feeding M., he starts crying and I start leaking. But there's just times and are going to be more times when I just can't get to him the instant he squeaks, especially with a 6 year old to take care of too.
If you can't get to your daughter on her crying nights, at least talk to her until you can get to her. She may not understand your words but she can understand your tone. If she's had her needs tended to then she's fine, on her crying nights work slowly, go in at first pick her up give her a cuddle till she calms, then leave, if she wakes again go in and pat her on the back and talk to her, the next time just stand close and talk, I sang. Little by little she'll catch on.
Good luck.