12 year old's do not want to act like young kids anymore. Here in Austin 12 year old's are in 6th grade which is middle school. Switching classes, advanced classes, football games, dances..
They are considered young ladies and are a little more aware of their bodies.
Could be she has now experienced her menstrual cycle and does not feel as comfortable sitting on anyone's lap, stepfather or even father or grandfather.
She is beginning to have some physical boundaries and this is a very good sign that she is following her internal feelings. You all need to honor this.
I do think as a family in general you ALL need to do things all together. Have a Game night, pizza night, attending religious services etc, bowling, putt putt.. ..
Also make sure your home is the gathering place so that she and her friends will want to be there to get together.. You will not be hanging out with them, but you will be around to get to know her friends and they can know you.. This is all very normal development.
When she is more affection or needs some attention.. you and mom, need to be there for her. But do not take it personally.
Just let her know you guys love her and you will always be there for her.