Hi C.,
Sounds like this is just a matter of boundaries. (I am a Life Coach and this is a very common challenge for many people (saying 'no' without hurting feelings or the relationship).
A very easy way for others to receive a 'no' (without getting their feelings hurt) is for them to first hear or understand (from you) that you would like to help them out.
Because your friends situation is different than yours (1 child, compared to your two, and married vs. your singlehood) she understandably (but a bit surprisingly) has no idea what YOUR reality is like (a bit maxed out, I would say).
My advice would be to say something like this: (Note: I put important words or phrases to include in your communication in ALL CAPS)
"I would love to help you out more (or babysit for you more), but with the twins and no other hands around here to help me, it is difficult for me, AT THIS TIME, to handle anything more. I am sure this will change as the twins get older...but for RIGHT NOW, I am not able to babysit your child (or can only sit for X amount of time).
I hope you can understand that I am doing the best I can, and what I need most right now is to keep my time, home, etc. manageable."
C., a lot of people are afraid to communicate directly what they need or feel. I don't know if this is you or if you have no problem doing so. From my experience, I can assure you, if you are not able to care for yourself in this way (or in other words...stand for yourself) the tension or stress in this particular area of your life is likely to get worse, until it eventually breaks and makes it more uncomfortable for you.
Regarding your friends who get offended when you don't immediately return calls...you could either change your outgoing message to say something to the effect that "I am not able to take your call right now...and, unless it is an emergency, it may be a few days till I can return your call.
Or, you can talk them directly and say.... "I know it is frustrating to call me and not hear back from me right away...I would be frustrated to..but right now, all my time is consumed caring for the twins, the house, and then myself. I very much want to talk/visit with you. The first minute I am truly free of any chores or responsibilities here...I will call you."
(Note: What your friends ultimately want is to know you still care about them and want to spend time with them. They love you and probably do understand (logically) that you are up against a lot. They just need to know you still think about them and want to spend time with them).
Hope this helps.
L.