For starters, quit spanking her. It's hard enough for children to adjust when two people with children get together.
Your love and attention is shared now with another man and other children and is sometimes hard for children to adjust to that.
Your husband picking fights with her and showing favoritism towards his children isn't helping her at all.
It isn't that she needs to adjust to having a new father figure and new brothers or sisters as much as it is that you and your husband need to work harder at helping her understand that there is plenty of love to go around.
All children need attention and reassurance of our love, and sometimes they feel that the only way to get your attention is by being bad or argumentative.
You and your husband need to be in counseling with her as a family unit.
All the therapy in the world isn't going to help her if she comes home to fights and spankings.