As long as he's ok with it? Really? Does he own her or something? I say go for it.
Spoken like someone who's never been married...or in a good marriage :lookaroun
To say "if he's ok with it" is not a question of permission, it's one of mutual respect. Aside from what some may consider petty reasons - maybe he'd be jealous she gets to go and he doesn't, maybe he'd miss her too much, and she'd miss him just as much were the tables turned but neither will admit it - there are other, practical, reasons it needs to be ok with a spouse for the other to go on vacation without the other. The two that spring most immediately to mind.
1: Budgetary. Unless the OP's relatives are paying for every aspect of her vacation, she may be spending money the family needs for other things...or t least, this is money that will effect vacation plans they'd take in the future - maybe when they go to WDW next time, it'll have to be for a shorter period of time, or not in the resort they'd like to be in, or eat in the restaurant they'd like to visit or get the souvenirs they'd like to get, or even delay the trip entirely. Even though the OP only works part time and is in a position to take time off whenever she wants, can the family, the couple, afford it? My ex-wife was awful with money (one of the many reasons why she's an ex), and near the end of our marriage, she'd often argue about things she wanted to do, things she wanted to buy, with "her money." I'd have to point out that, by her spending "her money," I had to cover all the house expenses (including her cell phone, her medical expenses, and her car payments) with money I earned, which somehow became "our money," and it wasn't fair.
2: Home responsibilities. There may be some things she does at home that the husband, quite frankly, can't. And I'm not talking "make my dinner, woman." Example: my commute is such that I usually leave the house at 8 and not back home until 7:30, at the earliest. If my wife wanted to take a vacation without me, and she wasn't bringing our daughter with her, I'd need her to help me arrange to have someone we love and trust pick our kid up from daycare & watch her, feed her, keep her safe & hopefully happy until I get home. Before we had a kid, but we had a dog, it was the same situation, I either had to find someone to help us take care of the dog, or I had to arrange to leave work early or work from home. And if I got to leave work early, I still had to arrange emergency backups in case I got stuck on the train (as anyone who takes NJ Transit can testify, that happens far more often than it should). For any one person in a couple to be gone, it can mean more responsibilities to the other.
Back to topic, I know my wife wouldn't be cool if I PLANNED a vacation without her, for those 2 reasons I described above. If I were traveling for work somewhere near a Disney Park (or anything else that interested me), and I had the time during that trip to go for a day, she'd be more OK with that. And if something happened that prevented her from going on a trip we already planned, and money was already spent, she might be more OK with that. And I'd pretty much be the same way. But our budgets and schedules are heavily intertwined, there are things I need her to do, and things she needs me to do. We're a team; for either of us to just go on vacation without the other would potentially be both an emotional and a financial burden.
But he never goes! I think he would feel bad if I weren't there. So it is a tough question. I know all of the memories I have from WDW have DH in them so I would have a hard time with him not there, but on the other hand if you are going with other special people in your life that would still be special...I see why this is a tough question! 
