P.M.
Imagine yourself receiving messages, subtle or otherwise, from a boss informing you of her impatience with you, and comparing you badly to other employees. It probably wouldn't inspire you to feel capable or confident, may leave you feeling hopeless, angry or exasperated. You'd eventually focus more on your worries about her feelings about you than on your job.
Your daughter probably knows your feelings of frustration, and how she compares in your mind to her sister. I hope you can sense how discouraging a frustrated mommy could be to a small child as she approaches a major developmental milestone. Potty training is complicated and challenging if you're not actually ready to master it yet.
If she's only two, she's on the early end of when kids are usually mature enough to potty train. For girls, most won't be fully trained until 2 1/2 –3 1/2 (though there are exceptions), because it takes that long for the nervous system to mature, bringing with it the ability to recognize urges and prompt the act of voiding (or hold back until a potty is available). This maturation occurs at different times for peeing and pooping for many chlldren, with pee success usually (but not always) happening first. Kids are all such individuals.
Before her body is matured enough, getting frustrated with her runs the risk of setting her back emotionally. Littles are thrilled with success at new capabilities, but discouraged and even soured by lack of mastery if the parents show disappointment or frustration. She's not ready to recognize a quadratic equation yet, and you wouldn't expect her to learn algebra. On a physical level, it may be a long while yet before she can learn to recognize the sensations around poop, and what to do about them or at what pace.
Whatever you can do to keep the process light and playful will probably work toward success in the coming months. My grandson loved role-playing with his toys while he was learning. Comparing your daughter to her sister and labeling her a "difficult" child may seem subtle enough to a parent, but are loud, negative messages to a child that will work against her willingness to cooperate.