It sounds like they may have had a really lovely time last year, and he wants to repeat it.
Does your DSD live with you? Does he get to spend much time just with her?
I think YABU, actually.
If you really want to go, you can queue up for returns, every afternoon, and see some tennis. (I know people who do this most years, and have a lovely time.) Or is it just that you think it's not fair that DP has chosen DSD, and not you?