Am I being a precious brat?

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We are going on holiday next week for one week. My birthday is on one of the days.
I had planned for the three of us (DH and DD) to do something for my birthday.
I've since found out my sister and her family are coming to the same holiday spot for two days.
One of these days is my birthday. Now we are meant to be doing things with them on both days. I like my sister but she is never on time for things and we have to wait around for her and her family to go anywhere and I always end up looking like an uptight misery when I want to make the most of our time away. She always laughs off her lateness saying she is "laid back".

I know I'm being a precious idiot but I really wanted to do the planned activity on my actual birthday. We have now changed it to another day.

What I would really like to do is just say no I want to do things with my husband and daughter on my birthday but I know I will look like a mardy cow.

Also it's taking up two days of our holiday and I'd really just like to spend time just the three of us. We rarely get the chance to all be together what with school/work etc.

I don't know what I want to actually ask now. After writing it all out I know I look like a knob...
If I'd known they were going this week I would have picked a different week for holiday tbh.

Go with what you want. Say that the activity couldn't be changed, if you still want to do it in your birthday. You didn't choose to holiday with them, so I don't see why your plans would change to accommodate them.

Someone being persistently late and trying to pass it off as being 'laid back' - to heck with that. Leave them to it. As pp said, crack on, let them catch up.

Enjoy your holiday and your birthday.

Well, you could always tell her you had already made plans for your birthday and you know she won't mind if you go ahead with them, as she's so laid back and won't get uptight about it, whereas she knows you are the sort to get uptight about changing their plans.

Go with your original plans. Dont change the timings but continue as if you werent expecting Dsis and family at all.
I hate pandering to these time wasters who are usually so laid back that silly old customs like using a clock for timings simply doesnt apply to them. They must just go with their own feelings about when its right to leave for an appointment.
Well let them I say. You stick to your old fashioned ways and enjoy your day. If it coincides with them drifting into your locality so much the better. But if not ,then what a pity - maybe next year ?

Revert to your original plans.

If you do end up planning to do something together and she is late, proceed at the original time. Then, if she proclaims that you're rude to start without her, tell her not to be 'so uptight'.

Don't let the selfish cow dictate your holiday.

I've already changed the birthday arrangements so don't want to mess venue about so will stick to the following day. As pps said it will probably only annoy me.
There isn't a surprise she isn't the type to organise things like that either.

I have taken on the advice about "cracking on" though. We are meant to be meeting at an attraction now on my birthday now and we will be there on time. We will just go inside when we arrive and they can find us inside.
Normally we would wait in the car park for half an hour with me silently seething while simultaneously trying to appear breezy to DD while waiting for them to show up.
Thanks mumsnet

We are meant to be meeting at an attraction now on my birthday now and we will be there on time. We will just go inside when we arrive and they can find us inside.

This is exactly what you have to do, every single time. I had a friend who was chronically late, and I just started to either go ahead without them, or leave (without sending a message saying I was). "Why did you leave?" "Well my company was obviously unimportant enough for you to be on time, so I didn't bother wasting my own time" or "I came all the way in and you weren't there" "Yes, it's very annoying when people waste your time and expect you to give up their time to be somewhere isn't it?. She started being on time for things with me because she knows I won't put up with that shit (she's still late with everyone else though). Don't let people walk all over you.

We will just go inside when we arrive and they can find us inside.

This is the true test of her laid back nature. If she kicks off, she's not laid back, just selfish.

Was she the 'golden child' growing up?

That's the way to do it, OP. And do that from now on, sail on with whatever activity, meal, whatever it may be, in future, at the time that was agreed, and let them catch up.

You will soon see how 'laid back' or otherwise she is. And if she moans you can say it's your turn now to be laid back and tell her not to be uptight.

Enjoy your holiday and have a great birthday.