With respect to the taking away from family time - don't get me wrong - I don't cancel plans to help her out or rush home from work and stay home to be on call. Even I'm not that much of a doormat ( and trust me I know I need to go to Doormats Anonymous )
I have said no to her before.
Where I am stumbling the most is the later in the evening request ( 9:00 - 9:30 ) when my kids are in bed, DH is heading shortly if he's not there already since he gets up for work very early and I am finally just sitting down after a long day ( I am sure most moms of younger children know what I'm talking about ). That's when I feel trapped like a rat since it isn't really cutting into family time when everyone else is sleeping ( or about to ) and she tells me I can just sleep on her couch so "I can get my rest too". I just feel like I'm at a loss for being able to say no - my mind just goes blank and I say my default yes.
This might be a little PA but I think I'll make a point of being "out" more past my bedtime - I won't like it but some things just have to be done...
You need to put yourself first. Why should you have to give up a good night's sleep when it isn't an emergency? And why should you have to give up some quiet time for yourself?
Next time she calls have your answer ready--write it down on a card if you need to and read it.
Friend: "I need you to come over and stay with kids."
You: "That won't be possible tonight. Hope you can find someone. Got to go, dh needs something." Then hang up the phone. If it rings again, do not answer!
Do this every time she calls. When she finally asks why you can't help, be honest and tell her you can only help out in a true emergency and not more than once a month. If you have already helped her once, keep saying no. You need to be in control of this for your well-being and that of your family.
In fact, if you have caller id, you do not have to answer the phone at all. After all you do need to use the bathroom, take a shower, be doing the laundry, talking with dh, checking on the kids, etc. You do not have to answer the phone unless you want to.