No, she did not believe it; she did not believe that Thomas Seymour was capable of treachery, of double-dealing.

But Elizabeth loved him; and she was young and beautiful, and a great future lay before her.

Catharine loved Thomas Seymour strongly enough not to want to deprive him of this future, but gladly to present herself a sacrifice to the happiness of her lover.
What was she--the woman matured in grief and suffering--in comparison with this youthful and fresh blossom, Elizabeth?
What had she to offer her beloved further than a life of retirement, of love, and of quiet happiness?
When once the king is dead and sets her free, Edward the Sixth ascends the throne; and Catharine then is nothing more than the forgotten and disregarded widow of a king; while Elizabeth, the king's sister, may perhaps bring a crown as her dower to him whom she loves.
Thomas Seymour was ambitious.

Catharine knew that.

A day might come when he would repent of having chosen the widow of a king instead of the heiress to a throne.
Catharine would anticipate that day.