she sounds so cute! thats a very unqiue mix of breeds

Is she spayed? that is one of the top causes of marking behaviors. If it's not a health problem and she is spayed, it is probably dominance related or insecurity. Crate train her, starting out by feeding her inside of her crate and slowly increase time inside. When you can't watch her inside, keep her in her crate. When you remove her from her crate, immediately take her outside to potty and reward her when she does. I would just frequently take her out, don't give her the oppertunity to mark the carpet, and be sure she isn't challenging you or your other dog with this behavior. She should go through Obedience classes with you, it's a very very good way to bond with a new pup and establish who's the alpha. I hope some of this works for you