Lying in a child of seven is usually not the intentional deception that we see in teens and adults.
Instead, the lying is used to reinforce a need. In this case, most likely, it is a need for attention as you suggested.
A child of seven is highly bonded to parental figures, and is greatly influenced by them, especially girls with their fathers. Coming into a newly blended family can be a huge challenge even for much older children, and at the ages of 5-8 years the difficulties can be more pronounced.This is due in part to the fact that a child of this age is not be able to express their feelings as easily as an older child, nor deal with the newness of the situation as easily as a younger child might.
As a result, the 5-7 year old child often turns to attention seeking behaviors. The more extroverted ones cause disruption and the average-introverted group uses passive means, like lying, to gain the attention that they need.
Using punishments is often inappropriate as a full solution as the emotional need will still remain. The behavior may cease, but the pain of the adjustment and the other feelings, including the very powerful feeling of jealously can motivate further problems.
Ideally, your daughter would greatly benefit from seeing a child behavior specialist. The feelings she is facing are difficult, and helping her express them well and in a healthy way is the best practice solution. Considering the complexity of the situation, I would recommend seeing a professional. This person will assist her to label her needs better and to use expression, not lying, as a means to deal with her feelings.
You may also wish to try something less involved, at least at first. A excellent resource is: Redirecting Children's Behavior by Kathryn J. Kvols, Bill Riedler, and Parenting Press (Paperback - Nov 1997)
This book, inexpensive on amazon.com, is wonderful and gives many ideas in how to reward and reinforce expression of her feelings.
Simply, her feelings need to be explored and expressed about her new family. At her age this is too overwhelming and comes out as attention seeking through lying.
Best solution is to seek support through a child therapist (her pediatrician can make a referral) as she is dealing with overwhelming feelings. Also, this may offer an opportunity for her younger sibling to learn ways to express herself, avoiding the development of any problems before they occur. Steven