Inviting 4 Friends In My 'birthday Party', Though They're Not Friends With Each Other?

[soah]

I have never been the type to get thrilled about my b-day, but this year I waned to do something different (I'll be 25 in one week). So I decided that I would gather 4 of my friends together for a spa treatment for all 5 of us and then go to a restaurant. That's all.
What might be an issue is that my friends only know each other because of me, and don't really have any sort of relationship with each other. To make it easier, I will call them the girlfriend, the bff, the sister (she is actually my sister) and the movie junkie pal.All of them have met the other before.
The girlfriend likes the bff and is ok with the movie junkie friend, but does not like the sister that much. It's not like she can't stand her, she just does not like her.
The bff likes the girlfriend and movie junkie, but has a complicated relation with the sister (kinda like the girlfriend).
The movie junkie is cool with all three of them. She is a sweetheart, I know. She kinda tries to make things work instead of picking them apart, and is the most understanding, tolerating out of them.
The sister does not dislike any of them, but I have a hard time understanding what she actually thinks.
I love all four, but kinda get *** of to the sister at least once a day. Our relationship is not very friendly. If she were not my sister, I don't think I would hang out with her ... But we are supportive of each other in time of need (except for once or twice when she has royally let me down, which is one of the main reasons why the bff and the girlfriend don't like her). Either way, she also has supported me many times, so I feel like I owe her.

I wanted to organize this thing to get together the people that have helped and supported me through my 25 years (those I still am in touch and that can make it. so childhood friends or people living in other countries unfortunately can't come). I guess it is about me thanking them (by giving them 2 hours of relax and some good food afterwards :) ).

But is it selfish to ask these 4 rather unrelated people that may or may not like each other spend a couple hours from their sunday together?
I would have separated them in groups, but to be honest (and a cheap ***), I cannot afford paying 8 bills instead of 5. Also, it is a bit time consuming, and I wanted to give them the same treatment, since in a way or another, they are all as much important to me (even my brat of a sister) and I wanted them all in the closest thing I will ever have to a birthday party.

[Jackdaw33]

What's a bff?

[Zacs-Master]

Best friend forever.

No one can possibly comment whether it will be a success or a failure as we don’t know them.

[Ken4155]

If they can't put up with each other for a few hours for you, then they're not really friends, are they?

[albaqwerty]

Best Friend Forever, jack

[soah]

@jack, bff is best friend forever, that's the name she uses to save my number on her phone. Also the longest friendship I have.
@ken4155: I am not saying they won't put up with this thing, but I don't want them to have to put up with anything. I want them to have some good time

[woofgang]

While I understand and salute your wish to give these four people a thank you...I wonder how much of a thank you it would be if they really do not get along or have to struggle to get along? If it was me I would look at my budget and work out what I could do within it that would provide one treat for your three friends and a separate one for your sister...and if that's not a spa and a meal then so be it.

[Tilly2]

Don't worry about it. They can sort themselves out. Enjoy your birthday.

[Jackdaw33]

You live and learn. Where did bff come from?