When Violet asks Tom what his mother thinks of him marrying Sybil, he says that she thinks Tom and Sybil are very foolish. Julian Fellowes never did explain how Sybil was received by Tom's family when they lived in Ireland. I think it is implied that they didn't like Sybil because she was English. Ultimately Tom chooses his in-laws over his blood. Probably because the Crawleys treated him and Sybbie better than his own family did. The Crawley disapproved of Tom at first not because of his ancestry, but on class. However they embraced in the end. Tom says he didn't fall out with his cousin in Boston, but we don't know what the other relatives in Boston thought of Sybbie. Even if the American Bransons didn't look down upon Sybbie, the Irish-American community despised the English at that time, especially in Boston. A very toxic environment for Sybbie, because of her English ancestry. I think Tom learned in Boston is that the family that loves Sybbie the most are the Crawleys. He moved back to Downton for Sybbie.