Are you already presenting as female sometimes? On HRT? Planning on transitioning? If you already are in the process of transitioning or about to start to, just honestly telling about it as soon as they get to know you might be a good approach.
On the other hand, if you don't want your gender to be the no 1 discussion on your holiday, you can at first get to know them a little better - and share the info once you're ready (the same way you'd 'come out' to any new person in your life).
If you are still fully presenting male and are okay with it, you could also wait until you know them a little better and feel more comfortable doing so.
But since you stated that 98%... It seems that you in your heart know you are a woman - but are a little scared to utter that aloud! If your only reason not to tell anything is because you are afraid 'you might later on change your mind' or something, that's a silly excuse for not being open about it, if you actually do want to tell.
Introducing yourself to your new family as yourself might make you feel much more comfortable in their presence! If you suffer from dysphoria or hate to be misgendererd for example, that might hinder your ability to develop a genuine relationship with your dad. If you are having a constant feeling of 'you don't really know how I actually am', that's a sucky feeling. Having a total fresh start with your dad (as you are having now) is also a good starting point to be open about your gender from the beginning. If, for whatever reason, your family members weren't accepting, I'm afraid that's a risk you must be willing to take. You must mentally prepare for a bad/not so good outcome, especially if you are having high expectations regards to your new relationship with him/them. If you are not prepared to risk it no matter what - you can just go there, get to know them as people first - and then later on tell more stuff about yourself.
I'd say follow your gut feeling! If you are feeling you'd actually like to tell, but are a little scared, just do it. If, while there, you don't feel it's the right time for it after all, don't. Or then you can make a decision in advance: you are going to tell them no matter what and that's the foundation for your new relationship with them. Or: I'm going to tell them only if I feel comfortable doing so while there.
If the only thing holding you back is the doubt, then I'd say just do it!!! You don't need to be 1000% sure on anything to say that aloud!!! You have the right to say 'I feel this way'. That's your reality as of now.
