Oh, what a bloody mess, Dinny
OK, I'll have a ramble and see what you think.
I think that once you have children, your relationship with your siblings alters. It has to. They are grown up, can fend for themselves and hopefully understand that you have little people who are dependent on you.
Then there are weddings. Weddings are peculiar things that bring out the very best and the very worst in people.
It seems that, sadly, this wedding has brought out the very worst in your sister. She has planned a date and a location that are nigh on impossible for you to attend (even without your flying phobia, Sardinia's still a strange choice, given your dad's health, etc).
If you want to stop bfeeding DS, this would be the time to do it. I know you'd wondered about going away before now, but I'm not sure that it being "forced" upon you by your sister is the way forward with it - resentment might build if it goes badly and DS is upset while you're away.
Then there is DD starting reception. I think she'll need you around. People have said it's not a big deal, but I think they're wrong.
I would ask her if there's any way she can change the date or the location. If she refuses, see in the meantime if you can think of a way to get there, but from her pov you have to tell her you can't make it.
It's such a shame - it should be something you can both look forward to - your sister getting married is a lovely occasion, but sadly it seems like she's just trying to put obstacles in your way. I wonder if she's subconsciously not wanting to wed this man and will blame your "lack of support" when it all goes tits up?
I seem to have spent ages saying not much. Sorry. Email me if you need to. Oh, and you're not at work next Thursday are you? Chance I might come in, I think. Can't remember when you work though?
And Stitch - touch of the bridezilla about your "my wedding more important than sister's medical exams that determine the rest of her life", I think