I know its just a vent, but I did put some advice in here so be warned lol. My IL's are good about giving us space, but DH does a great job with boundaries.
You guys are way too easy going. Since you already agreed to the MIL and SFIL vacation then I guess you have to go. But DH needs to talk with the Uncle and let him know you are unable to attend. Do they pay for you- is that why they are booking it for you? It is between you and Uncle on the trip he is planning, so MIL should not be involved- meaning don't let her be involved. If she gets mad, let her get mad- she'll get over it and then know you mean it when you say you are not attending.
As far as plans with the kids, stop telling her about your plans and then she can't invite herself along. Or tell her or have DH tell her it is immediate family only since you never get to spend time that way. Or you can occasionally be "sick" for family events, or send the kids and take time for yourself and do a "me" day or a "girls night out".