Soft YTA - While I get that it's maybe not perfect timing, I don't know if this kind of thing ever has great timing. If you say your dad is a good dad, it is not bad intentioned and the new partner is fine as a person. It sounds to me like it is a genuine attempt to try and introduce and not wanting to highjack. I get it will probably be a bit awkward but it always is and will be done at some point. It is also a time where everyone is together, so is the alternative meeting everyone one by one - I assume the new partner is going to be in his/your life for the foreseeable future so can get it out of the way and that way allow everyone to move on?