Without boring you with all the details, I made a recent connection between one of her brothers, Nicholas, dying almost immediately he stepped off the ship in New York in 1872, in what turned out to be the marital home of another sister, Susanna. Researching Susanna and her family (which curiously included a man who was buried twice!) I traced her back to an 1860 US census ... and lo and behold, who was living with her but sister, Sarah Seabrook! I'd probably found the record before, but as she was lodging within a household with several surnames, I didn't think it was conclusive proof that I'd found my Sarah Seabrook.
However, the moral of the tale is that I should have been more tuned in to the wanderlust of this particular group of siblings and have realised that once one had gone to the US, there was always a likelihood that others would follow and join their family out there at least to begin with. In fact 4 out of the 6 siblings ended up in America. So, if you do find a candidate for your relative in an American census who appears to be lodging with a differently named family, work them back to see if you've stumbled upon a big sister.