My thought is that there is a difference between a fully launched adult and a 19 year old college student. 

So if vacation is a lazy week at the beach or at a mountain cabin,  I'd probably ask if she wanted to come. Most college students wouldn't.

But, if it's a trip to some place geared to children, she doesn't get invited. The activities that are for a young child are not the same as a nineteen year old. 

However, that may be irrelevant. She's a college student. Shouldn't she be working during the summer and not on vacation?

The sd’s live in another state on their own, ss lives with us when on university breaks.

i am from overseas so every year me, hubby and our toddlers go, skids do not as they are not my family. Any weekend getaways or work trips, skids do not come. Ss stays home or goes back to university.

for new years we go back to hubbys family home in another state about 3 hours away or family weddings interstate and only ss comes if available.

since sd’s re-initiated contact with hubby mid last year, they ignore him unless they need favours or money, hubby will message them asking if they want to go to their cousins wedding and their response is “only if you want us to”. If the sd’s are coming i refuse to go with my toddlers. 

We’re expected to detour 1 hours drive away to pick the up when they’re perfectly capable of making their way. Any special occasions like our birthday or anniversary is just me/hubby and our toddlers. I do not entertain hubby asking where we go with his kids. He did this late last year for our 4th wedding anniversary asks me from work where i wanna go and to book tickets and skids will come along....

come again? Trust me hubby got the bitchiest wife ever that day. I will not spend our 4th wedding anniversary with 3 rude disrespectful skids sitting there in awkward silence, going on and on about bio mum and their stepdad, or pretend me and our kids do not exist. I shut that down real quick and told hubby if skids are so important that he stupidly wants to invite them to come for our anniversary, he can go on a holiday alone with them and me and our toddlers are flying overseas to my country courtesy of hubby’s credit card. Hubby said no he won’t ever go on a holiday alone with skids because its absolute torture because he knows how they treat others like shit and it pains hubby that the sd’s especially go out of their way to go on and on about bio mum and sd like they have to drill into hubby she still exists.

i will not go on a holiday with skids where they waste it non stop about bio mum this and bio mum that. Hubby told ss that time he’s unavailable to take him for a holiday. Yup ss messaged hubby 2 days before our 4th anniversary stating he was available if wanted to go somewhere with sd’s which hubby several months before claimed he was unavailable to go somewhere for our anniversary and suddenly a skid messages you and bam available!! I felt so undervalued and disrespected then 

Went once with SS11 and never again. I only travel with DH and our two kids. You don’t have to take your sd let your Dh take her on a solo vacation if he wants.

If it were your daughter living at home and going to Community College, would you expect her to be included? Whatever your honest answer is to that question, that's what you go with. 

That's a tough one. I wouldn't want to purposely exclude her. If she wasn't there full time, you could conveniently schedule it when she won't be there. Since she's older, hopefully she just won't want to come in the first place. And I don't see Disney being an issue - I can't imagine a 19 year old wanting to go to Disney with little kids. So you should at least get a break on that trip!

We have taken my BD (25) and grand with us (I have custody of my 7 yr old gd) to Florida for years.   She usually brings a friend with and they get their 'girl' time but they also watch the GD so we get some adult time.   It can work, but she is a wonderful Auntie and loves my GD.