Weddingbee boards

So awhile ago my aunt offered to throw me an engagement party on my grandmother's behalf (my grandmother is quite handicapped and can't really organize a party).  For about 4 months, we went back and forth about the party and finally picked a place and caterer and came up with a guest list. 

Shortly before invites went out, my dad and my aunt got into a fight about the party and my parents not being involved enough, etc. My aunt implied to me that she wanted me to choose between her and my dad and beg her to throw the party, etc. I was unwilling to do it, so I let the whole thing go and my grandmother said to just let my parents throw the party.  I did because it made them happy and hoped eventually everything would blow over.  It didn't really, and things have been tense.  She also happened to schedule a trip out of town the weekend of the party.  The party was last weekend and it was a ton of fun - everybody had a great time and especially talked about what a nice place the venue was.  

I cannot decide whether a thank-you note is in order here.  I am genuinely grateful to said aunt for picking the right venue, for helping organize initially, and for offering to throw the party at all. (Which my parents did not offer to do, but let me know four months later that they felt left out.)  I also genuinely understand that her relationship with my father is extremely tense and strained, and I'm not particularly surprised by what happened between them.  I also do not really fault either of them for this, it is simply the way things go in our family sometimes.  

I would like to write a note that says thanks for starting the process, etc. but I feel like it will just look petty - and I don't want to do that - but I do want to let her know that despite all the crappy stuff that has happened later, I really appreciated the party and it wouldn't have happened without her starting it.  How do I do that without saying "thanks for nothing!"  <span style="white-space: pre;">