Complex sentence: Nell's mother has "more hair than wit": she tells Nell that Cardross wants an heir and wishes to marry into a good family, and that Nell must be a conformable wife and not trouble Cardross.
Split into simpler sentences: Nell's mother, who has "more hair than wit", tells Nell that Cardross wants an heir and wishes to marry into a good family. She also tells her that she must be a conformable wife and not trouble Cardross.