Things like that are learned behavior and they don't learn it overnight. What are you going to do a year from now when she's still doing the same thing? I had a friend who's ds is the same age as my oldest ds and she was the same way, planned every little trip around him and to this day, 4.5 years later she's still doing the same thing because otherwise he has major meltdowns.
That's a pretty harsh view. 1 year is a huge difference to a baby in growth, communication, and understanding. It's easier to work with a 15 month old than an infant.
At 4 1/2 years, obviously the mom isn't wanting to change anything with her son's behavior and has
chosen to indulge him at her expense. I don't think either situation has anything to do with a 3 month old at an age where all experts now say you cannot spoil. She's uncomfortable and not happy -- the only method of communication she has is to scream.
My dd needed lots of nursing due to prematurity, extreme growth, and reflux. She was at least happy in the car, but at 4:50pm no matter how much or little she was in the car, she had enough of the carseat and would scream. At that time of day, she wanted ME, to be held, and nursed. Nothing else would suffice.
She absolutely terrified my inlaws on a ride back from an outing when she was about 10 weeks old. They were riding in the back seat on either side of her. I had a complete demon child -- Red face and full volume for an entire hour. No matter what comfort they offered, she ignored it and kept going. I had nursed her for 20 minutes before we left, but she had reached her limit of being on her own. She's grown out of it since, but communicating her needs has become easier with her maturity. In fact, the automatic 4:50pm time clock whistle evaporated almost overnight when she reached 13 1/2 months -- when she biologically turned 1 year old.
Since your 3 month has not liked car seats from the start, you might want to find out exactly why. Have you tried sitting next to her while someone else drives to determine if it's the separation from you that upsets her? The other could be that she doesn't like the car motion and is getting ill. The straps also might be pressing on her stomach in an area that makes her sick.
hth and I hope you find out something that can help.
