The tale opens in Boston and New England in the middle of the 19th century, and describes the experiences of two European siblings shifting from the old to the new world. The two protagonists are Eugenia Münster and Felix Young, who since their early childhood have lived in Europe, moving from France to Italy and from Spain to Germany. In this last place, Eugenia entered into a Morganatic marriage with Prince Adolf of Silberstadt-Schreckenstein, the younger brother of the reigning prince who is now being urged by his family to dissolve the marriage for political reasons. Because of this, Eugenia and Felix decide to travel to America to meet their distant cousins, so that Eugenia may "seek her fortune" in the form of a wealthy American husband.
All the cousins live in the countryside around Boston and spend a lot of time together. The first encounter with them corresponds to the first visit of Felix to his family. Mr Wentworth’s family is a puritanical one, far from the Europeans' habits. Felix is fascinated by the patriarchal Mr Wentworth, his son, Clifford, and two daughters, Gertrude and Charlotte. They spend a lot of time together with Mr. Robert Acton and his sister Lizzie, their neighbours.
Eugenia’s reaction after this first approach differs from Felix’s. She is not really interested in sharing her time with this circle. She doesn’t like the Wentworth ladies and does not want to visit them frequently. In contrast, her brother is very happy to share his spare time with Charlotte and Gertrude, spending hours in their piazza or garden creating portraits of the two ladies.Eugenia and her brother Felix arrive in Boston. The next day Felix visits their cousins. He first meets Gertrude, who is shirking attendance at church. He stays over for dinner. The next day Eugenia visits them. Three days later their uncle Mr Wentworth suggests they stay in a little house close to theirs. Felix suggests making a portrait of his uncle. When Mr. Wentworth refuses, he makes plans to do a painting of Gertrude instead. The latter walks into Mr Brand again and bursts out crying when he asserts that he still loves her. She then sits for Felix to do his painting of her, and he reproaches his American relatives for being very puritanical.
Eugenia is talking and flirting with Robert Acton; she says she will divorce her husband. She visits Mrs Acton and says a white lie - that her son has been talking about her a lot - which comes across as a terrible faux-pas. Later, Mr Wentworth tells Felix that Clifford got suspended from Harvard owing to his drinking problem, and that he is improperly in love with Lizzie Acton - Felix suggests fixing him up with Eugenia instead. Later still, Gertrude tells him her father wants her to marry Mr Brand, though she doesn't love him. Mr Brand then criticizes Felix. Gertrude emotionally blackmails Charlotte into keeping him from talking to her, lest she tell him Charlotte likes him. Clifford then visits Eugenia. Robert Acton goes to the Wentworths' but Eugenia is not in their house; he goes into hers and asks her about the divorce note and going to see the Niagara Falls with him. Clifford comes out of his hiding place; the two men get back together.
Felix tells Eugenia he wants to marry Gertrude; she admits to being unsure of Robert. Mr Brand then visits Felix, who tells him Charlotte likes him. Eugenia gives her farewell to Mrs Acton as she prepares to move back to Europe. She walks into Robert, who says he loves her - she has sent the divorce letter; he will have to join her in Europe. Later, Felix asks Charlotte to tell her father he would be a good prospective husband for Gertrude. He then meets with his beloved again, and she says she would leave her family with him.
Three days later, Felix decides to visit his uncle and tell him he wants to marry Gertrude. The latter turns up and tells her father the same thing. Mr Brand asks for Mr Wentworth's consent to marry Gertrude and Felix - he agrees. Mr Brand and Charlotte later marry. Clifford has proposed to Lizzie Acton; Eugenia, however, has repudiated Robert Acton, not actually signed the divorce note, and is traveling back to Europe. Years later, after his mother's funeral, Robert would find a 'nice young girl'...

Question: Who is Felix to Eugenia?
Answer: It is her brother.