AIBU to think you don't invite other people on days out you have planned weeks in advance?

[WeAreEternal]

I arranges to go shopping with my sister today, I took the day off work especially.

We live a couple of hours away from one another so we don't see each other that often.

We arranged this trip over a month ago, it was her idea.
We both live around 30-40 miles either side of The city that we are arranging to go shopping in.
DSIS doesnt drive so planned to get the train (it's only a 20 minute journey) and meet me there.

Last night I called her to see what time she wanted to meet up and she said "I've arranged a lift so can meet anytime"

I asked her who the lift was with and she said a friend who I don't particularly like but get along with
I asked her why she didn't tell me that she was inviting someone else. She said that she didn't think it mattered.

AIBU to be annoyed that she invited someone else just so that she didn't have to get the train?

I wouldn't have minded so much if she had told me before hand, but she has just left it to the last minute (not unusual for her).

Plus I hardly ever get to see my sister, the last time I saw her was in the half term.

Bit naughty of her, I'd cancel too and go on my own

It's mainly a bug deal because we hardly ever see each other and I don't really like know this person, so with them I will feel like the third wheel on our day.

Cancel. Do whatever prep you need for christmas instead, everyone at this stage of the year has a long 'to do' list, get it done. Tell her that it's because you wanted a sister day out, if that's not happening you've got other things more pressing than a general shopping trip (which I can't see being fun with someone with no money to actually buy anything, why does the friend want to come watch you shop????)

Your sister will get the message. I'm sure you'll see her over christmas.

YANBU, i'd be a bit pissed off too. A few weeks ago, I met up with a friend and we took the train to the city, left all the kids at home with the DPs who got together and took all the DC swimming and out for the day. Friend and I had a lovely day, shopping, lunch, bit more shopping, pub, then home where DPs had made a lovely curry.
It was a really nice day, spent with a friend I don't get much time with alone. She's great company and I love spending time with her. If she had bought a friend with her it would have been a totally different day and although would have also been fine, it was much preferable to just go together.
I realised afterwards just how much I had been looking forward to our day out.

I would just cancel. Days off are precious, and not to be wasted like this. You took the day off to spend with your sister, she will now be spending that time with her friend and you would feel like a tag-along gooseberry. No. Just, no.

Plus, maybe if you cancel, she will finally take the hint that she shouldn't do this to you again.

UPDATE.

After I dropped DS at school I called DSIS from the car, I told her that I didn't want to spend the day with her friend and id meet her for coffee but would shop alone if she wanted to spend the day with her friend.
She said she would speak to her friend and call me back.

She decided to get the train through and shop with me then have lunch and since I have to leave soon to go pick DS up from school her friend has driving through to meet DSIS so that they can shop together after I have to leave.

It worked out for the best in the end because the friend didn't get out of bed until 11.30 anyway.

Oh good, and thanks for the update!