Surprised that nobody has mentioned that jeans are maybe more comfortable and you can do a lot more physical activities in them.
And have you ever visited her school to see what the other kids are wearing? If most of the kids are in jeans - guess what she is going to want to wear?
And it is very normal to copy your sibling. It has got nothing to do with her sexuality. Let her wear what she wants. Save your energy for when she is in high school and wants to go to school with hardly anything on :)
I also want to clarify the comment "try not to worry, it's probably not a signal whether she is gay or transgendered," doesn't mean I think you should worry if she does turn out to be gay or transgendered. I don't think a 4-year-old calling herself a boy when wearing jeans signals her future sexuality. But if it were, all the research says being gay or straight is not learned, it's inborn. Your job as her mother is to love her exactly who she is.
Oh, also, you sound worried that she calls herself a boy when wearing pants. Try not to worry. I don't think it's some kind of signal that she is gay or transgendered. If she says it in front of someone, don't be embarassed, kids say stuff all the time. Just roll your eyes and smile and let it go. Certainly you aren't going to change her sexual identity by allowing or preventing her from wearing jeans and a t-shirt when she is a toddler. My little sister used to insist she was a character from a set of books she had called "Cowboy Pete," and when she was dressed in cowboy boots and jeans, she would correct us if we accidentally slipped up and used the female pronoun about her instead of the male, and she would raise a fuss if we called her anything besides Pete. I don't think she was much older than 5 when she went through her Cowboy Pete stage. I think it had to do with feeling free from the connection she made between being female and having to live a more restricted life in terms of clothes (she hated frilly stuff) and behavior.
Let her choose her clothes. Kids get little enough control over things in their life; some adult is always standing there telling them not to do something they want to do, or to do something they dislike. It will hurt absolutely nothing if she wears jeans and t-shirt every day from now until she is 21.
Your daughter is only 2 years of age. I'm sure that her choosing boys clothing has absolutely nothing to do with her gender or gender preference. Like all two year olds, she is experimenting and attempting to control her environment. For now, she may simply feel more comfortable in boys clothing. I would not be concerned about this behavior.