Yes you can take one and get on with your day, id suggest one 25mg tablet, but if you're unsure, then as long as the tablet has a dented midline you could start with half a 25mg and if no difference felt after an hour, then take the other half.
Also rescue remedy pastilles are good. You can also get 1 use ice packs you just snap. But check rules around these if flying, these are good for any panic attack type moments, placed on the back of the neck, wrists.
If you can, luggage wise, maybe make yourself a mini MH first aid kit, ice pack, rescue remedy, a note to yourself or written by someone you love, a photo you cherish, a bar of chocolate or a favourite sweet, a Spotify playlist, I have playlists called "drown it out" when I need to stop impulsive thoughts, "meaningful songs".
You could play "family gathering hell" bingo. Write your own bingo card of things your expecting people to say or do or situations. And then tick them off. It might help by turning your mind from ruminating on whats said to "oh another one down" and then you can focus in your mind when people are saying or doing things "bingo".
If theres any point you wish you could say "fuck off" but you cant, I find it helpful to use my tongue to write the letters "fuck off" on the roof of my mouth. No one can tell, you can even do it with a fake smile on your face.
And plan something really nice for yourself for once its all over. Whether that's giving yourself a day to wallow in self pity with chocolate and films, or going out somewhere.
Just my suggestions, apologies if ive overstepped