So backstory: late last November, shit went down. My dad's wife is an alcoholic, and because she's incredibly immature, we (that is me and FI, my sis and her FI) knew that any sort of intervention we tried to stage would just turn into a screaming match. Instead we sat down with dad and calmly and rationally told him we were worried for stepmom, for him, and for her kids, that we didn't feel safe or welcome in their home, and that we would appreciate it if stepmom would treat us as friends or at the very least adults, as we (me and sis) were both adults when they started dating and didn't need a second mother. It all went very well, or so we thought.
The next morning he emailed both of us to say that he felt we had ambushed him, that our fears were ungrounded (basically that it's all in our heads, despite several other people saying that they're no longer willing to socialise with stepmom), and that we need to act like adults before we can be treated like adults (WTF?). At the end of the day he basically said that seeing as he clearly couldn't have both us and stepmom in his life, he'd rather have her. Fine, fair enough, he's entitled to that choice.
So now comes the tricky part. My sister, who is getting married in June, has decided not to invite him to her wedding at all. Again fine, it's her choice. It's her second marriage, and she's only inviting 30 people - I'm the only member of the family who's invited, and I think it's probably because I'm the only member of the family who hasn't told her that she's making a mistake or expressed a dislike of her FI. The way I see it, she's paying for her wedding, she can invite or not invite whoever she likes.
I toyed briefly with the idea of not inviting dad to my wedding (October), but I really can't do that. It just doesn't feel right. Not to mention the fact that my grandparents (mom's folks) will want to know why he's not there, and if I say "I didn't invite him" I end up looking petty. I want them to see me in a good light. So FI and I decided that we would invited him and I would include a handwritten note with it, explaining a) why we're inviting him, b) why we're not inviting stepmom, and c) that nothing is expected of him but his presence would be appreciated.
Now here's what worries me: if I wait until closer to the wedding to write the note, I will probably be extremely emotional and end up saying things that I don't mean because I'm angry at him. Do you think I can draft the note now, and just write it out when I send the invites? Are my fears ungrounded? Am I totally overthinking this?