Complex sentence: Although Isabel had previously rejected both Warburton and Goodwood, she accepts Osmond's proposal of marriage, unaware that it has been actively promoted by the accomplished but untrustworthy Madame Merle, another American expatriate, whom Isabel had met at the Touchetts' estate.
Split into simpler sentences: Although Isabel had previously rejected both Warburton and Goodwood, she accepts Osmond's proposal of marriage. She is unaware that this marriage has been actively promoted by the accomplished but untrustworthy Madame Merle, another American expatriate, whom Isabel had met at the Touchetts' estate.