Hi Y., This sounds really familiar to me! I wonder if your daughter isnt just one of those kids like my son who tends to be really sensitive to the world around him.
The things that key me into this are that you say she seems more sensitive and less emotionally mature than other kids her age, she whines and it happens moreso at bedtime when she is tired and the addition of a new family member seems to maybe be making it worse. As the mom of a "sensitive" kid I'll tell you that when they are over-stimulated, they can be pretty annoying.
As adults, we learn to filter a lot of things going on in the background of our world and just focus on what we need to. A kid who is sensitive sees and hears and feels and smells every thing around them and it can become overwhelming and they live in a high-alert state emotionally which leads to the behavior issues and the whining, etc. We might have a small reaction to stimulation but their's seem huge and out of proportion.
Think of it as a bank account. Stimulation like going to school, homework, shopping, parties, crying babies, stinky diapers, poor sleep, TV, computer games all make withdrawals.
What makes a deposit are things like Gross motor movement, Motion and deep pressure stimulation - running, jumping, lifting, pulling, pushing, chewing, sucking, hugging, hanging etc.
If our kids are running at optimum and their sensory accounts arent getting depleted, then they can have mostly good days without all the whining and behaviors etc. When they live over-stimulated, then they just dont have the emotional resources to deal with stressors during the day.
Here are a couple of things we've done and it's made a huge difference - we have our "nice" son back rather than the pesty irritable one. If they help and you notice less whining, etc. - then you are on the right track with over-stimulation issues.
1. Turn off the TV and limit her computer and video game time drastically. Right now, my son gets 30 min of TV and 30 min of game time a day. Were going to cut that down further to 1-2 hours a week total once homework and sports start.
2. Lots and lots of exercise. Go for 2-3 gross motor activities a day. (running, jumping, biking up a hill, hanging on monkey bars, climbing, vacuuming, cleaning windows, pogo stick. Consider enrolling her in a sport.
Give her oppt'y to exercise before she gets overstimulated. My son takes a jog with my husband around the block before school and he gets to have playtime riding his scooter or bike and a snack before sitting down for homework.
4. Give her little doses of "sensory snacks" to make little deposits. (bunny hop to the car, pushing stroller, sipping from a water bottle, chewing gum or taffy, watering plants with a watering can, squeezing a stress ball or rolling modeling clay, build a blanket fort together and then read books inside, etc.
5. lastly, have fun. She doesnt even have to know why you're encouraging her to play. Discipline will only get you so far in this. If she really is having problems with stimulation issues, then she isnt even in a place where she can really hear you or do anything about it. The discipline becomes overstimulating in itself.
Good luck to you and your daughter. I hope my experience can help you in yours.
C.