should we reatain 7yr old twin just for maturity

[jtsbm4]

I have boy/girl twins they turned 7 in Nov. and are in the 2nd grade, they are 6mo behind their peers in age. They were both going to repeat the 2nd grade due to maturity reasons but now with 30 days left of the school year, my son is already doing 3rd grade work and is the top of his class. My daughter is doing 2nd grade work and is making progress at a slower rate. the problem is my son is going on to the 3rd grade and the school and I are trying to decide about my daughter. She is acting more like a first grader when it comes to her peers and school and at home. She crys about everything and is very shy to where she wont participate in fear of being made fun of. she is seeking attention anyway she can get it. She has stumped my husband and I for she got rid of all her toys except a few things, she chucked them outside her bedroom door and told me to sell them they are to babieish for her. We do not let her watch the tv programs that are not for her age but she is trying to be older than what she is and in the process looking and acting more younger without relizing it. She will not take my opinion on anything as far as clothes and toys and what to do. If I try and help her decide she will choose the opposite of what I have chose. I wanted to try and give somekind of insight to how she is. but my question is should we hold her back a year while her twin brother goes ahead.  mentally she is more like a first grader in my eyes but she is holding her own with school work and she is just as big as her peers. I do not want to ruin her self confidence anymore than for somereason it is. what is your opinion on this? Thanks