The bosmer had to be conquered to join- and they're neighbors. No, the mer have no love lost for one another (or any other race, really). I'm not saying that the falmer would be citizens of the empire, but neither would they join the dominion, except in the circumstance of absolute necessity.
What are you basing this on? More mannish propaganda? This may come as a surprise to the barbaric, hypocritical, genocidal, padomaic, violent Nords; but the Thalmor TRULY believe that they are helping their fallen Merrish brothers. They come off as extreme because the Nords(whom the pc is forced to see through THEIR perspective) see's them as evil.
There is more than one angle here. Try to keep that in mind.
EDIT: And no, the Bosmer were never conquered. The majority of them freely joined the Thalmor. The purges were directed to a dissident minority. That does not equate to conquered.