I think a large part of the blame for the breach lies with Mrs Norris. She could not be satisfied until she had written the long and angry letter to her sister, threatening her with all the possible ill consequences of her marriage.
She then made sure that those ill consequences would occur by making sure that France's (presumably predictable) bitter response was fully shared with Sir Thomas & Lady Bertram.
Add to this the other examples of Mrs Norris' "spirit of activity" which we learn about in chapter 1, and I think it is pretty clear that she is a thoroughly poisonous woman.