I know they each entered into this situation of their own free will, but I feel for both Mery and Janelle. I just don't see how being involved in a polygamous relationship can do anything but hurt your self esteem.
Meri was raised with the idea that her worth is measured by the number of children she bears. She was very, very young when she married Kody and must have been devastated when it took so long to conceive. Janelle comes in and immediately starts popping out babies. Meri's issues with fertility certainly were not Janelle's fault, but Meri's resentment is only natural. I probably would not have been the nicest person in the world to Janelle either, particularly at the age of 22 or 23. In a monogamous relationship, infertility would have been both Meri and Kody's problem, but in their relationship it became Meri's problem alone. (Similarly, today, the "empty nest" is Meri's problem.)
Janelle walked into a hornet's nest. Perhaps had Meri had a child first, things would have been different, but that was not the case. Janelle must have felt like an interloper from day one. Meri and Kody already had a home and a routine. Janellle never had the chance to have her husband all to herself. She never had the joy of being someone's true love--of being THE ONE.