Re: 3 1/2 mo dropping on the weight chart
No need to panic. Everything you describe sounds fine. Your baby is being fed on demand, she's not sleeping through the night, you feel plenty of letdowns and hear plenty of swallowing, she has normal diaper output, her development is as it should be, and her sister is small. Sounds like you just have a small baby, and you don't need to let fluctuations on the charts throw you! It's not like babies "fail" the growth test because they're on the lower end of the charts. Healthy babies come in all shapes and sizes, and statistically speaking, there are just as many healthy babies in the 11th percentile as there are in the 89th.
Is your baby very active? Doing a lot of reaching and kicking, maybe even some rolling? Highly active babies tend to "lean out" on the charts somewhat earlier than their more sedentary peers. Both my kids dropped percentiles between 2 and 4 months- of course, no-one ever made a big deal about it because they were roly-poly babies and I think the doc was secretly relieved to see them a little less obese.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"