Complex sentence: One potential stumbling block to the marriage was her step-grandmother's (Harriot's) will which forbade marriage to an alien, and as Bartlett was an American the marriage would cause her to be disinherited.
Split into simpler sentences: One potential stumbling block to the marriage was her step-grandmother's (Harriot's) will which forbade marriage to an alien. As Bartlett was an American, the marriage would cause her to be disinherited.