What to do....what to do... (long)

I have created a bit of a dilemma for myself, and now I don't know what to do.  It's a good kind of problem to have, I guess, but a problem nonetheless.

My mother is having her 75th birthday next year - she was born Sept. 29, 1935.  She is still an active, vibrant, fun-loving, creative and adventurous person who loves visiting new places and meeting new people.  My father, on the other hand, could politely be called a curmudgeonly stick-in-the-mud.  He hates traveling, dislikes being around people, and generally sticks to his comfort zone of books, TV, and online crossword puzzles.  My mother is also the youngest of 12 children, and one by one, her siblings have passed away, leaving only my mother and one sister remaining.  Several of her older siblings have had surprise parties planned for them on big milestone birthdays, attended by dozens of grandkids, cousins, nieces and nephews.  The thing is, though, that most of her family settled in the same general area, whereas she has lived hundreds, if not thousands, of miles away from them her entire adult life.  So naturally they are all much closer to each other than we ever have been.  I would love to have a big celebration for her 75th and invite all of them, but it's not reasonable to expect them to travel 3000 miles for it.  Still, I'd love to do something special to commemorate the occasion...who knows how much longer her health and vitality will hold up?

So I hatched a plan...and decided that this is what I would do with the earnings from my new job.  I decided to take my mom on a special trip - just us girls...someplace my dad would never take her that I know she has always dreamed of going....someplace beautiful and relaxing and really special.  I decided to take her to Kaua'i.  Okay, before you say anything -- YES, I chose Kaua'i at least in part because I also love it there and would love to go back!  But when I went for the first time in June '08, she must have said 1000x how much she would love to go there.  So....one stone, two birds, right?  If Kaua'i ended up being beyond my budget, other places my mom would love (and closer to home) would be the northern California wine country or maybe a spa in the Palm Springs area.  But then I got to thinking...my sister-in-law would love to go too...she never gets to take any trips unless they are work related or someplace my brother wants to go for his extreme sports adventures.  Then I realized that I could not ask my sister-in-law without also inviting my sister. I really detest spending time with my sister (sorry, but I do), but if we did this without including her, I would never hear the end of it.  She has no money, so I would have to pay her way.  I figured my SIL and I could split the cost...each of us pay 1/2 of the cost for my mom and my sister.  My brother makes a ton of money, so the cost is no issue to my SIL.  My mother would be thrilled with the idea of spending a week with her daughters...we've never done anything like that.

So I proposed the idea to my SIL, and she LOVED it. We agreed that it would just be us girls...we would have a blast.  She said she would defer to me on choosing the location and making arrangements....she would just pay 1/2 of whatever it was.  Great!  The only hitch is that she teaches at a private school, so we'd have to work around her school schedule.  They don't hire substitutes, and she also teaches summer school, so that leaves us with a very small window in which to plan.  We decided we wouldn't tell my dad or my sister anything until it was all set...he would just try to shoot us down and my sister would just blab to my mom.

So, the first warning sign came when my SIL told my brother about the plan.  He loved the idea, but immediately started trying to influence the choice of location....he said we should take her to Spain to see where her ancestors came from.  I have NEVER heard my mother voice a desire to visit Spain.  She would probably love it, but it's not anything she has ever mentioned to me.  As much as I would love to, the cost and logistics of that kind of trip takes it to another level entirely...one that even my fit and active 75-year-old mom might not be able to handle.  It's certainly not a trip I could afford.  Not to mention things like having to get passports, VISAs, etc...nothing you can do on the sly.  Then I found out that my brother is taking the month of July off next summer and doesn't want my SIL to travel at all in July.  That leaves us basically with a 2-week window to plan.  Furthermore, my SIL informed me that she has no intention of leaving her daughter, my 7-year-old niece, home for a week, so she would be bringing her along. 

So now my relaxing, special birthday trip for my mom has turned into a vacation with 5 people, one of which is my extremely annoying and very high maintenance sister, another of which is my wonderful but neurotic and slightly passive-aggressive SIL, and a third of which is my adorable (but 7-year-old) niece.  Forget relaxing dinners with a bottle or two of wine...we'd have a kid along who gets bored and antsy and needs to go to bed early.  Forget leisurely visits to the botanical gardens....my niece would be bored silly and cranky there.  Forget a relaxing sunset dinner cruise along the coast...my crazy sister would probably try to jump overboard and would drive us all nuts.  Not to mention that it has to be done during high season and not around my mom's actual birthday, because that is the only time my SIL can go.  Plus, how could I tell my own kids that they can't go if their little cousin is going?  How would that be fair? 

So I've gotten myself into this stupid pickle and I don't know how to get out.  I want to tell my SIL that I have changed my mind about the "all the girls" concept, and I just want to take my mom by myself.  But how do I do that without hurting her feelings?  As it stands, my original concept has been bastardized to the point that it sounds totally unappealing to me.  I don't know if I am being too sensitive about it or what....I know that I DO NOT want to bring any kids along, and that is non-negotiable for my SIL.  If I had just stuck with my original idea of taking my mom by myself, none of this would have even come up.  Ugh.