We have vacationed with half of my in-laws (DHs dad and stepmom) several times. It does take a lot of time coordination. We went to WDW w/them last January and Las Vegas last October. WDW was great, we were able to find a time that worked for both of us. Las Vegas, well, I bent over backwards to accommodate their schedules, so I was then disappointed that they wanted to spend more time in the casinos than hanging out with us. The next time we vacation with them, I'm going to have more to say about both the timing and the destination!
Other than ST and LM not being open, does April work for you? If it does, I would seriously consider going when she can go. You said that you've tried to get her to go in the past, so I'm assuming you would actually like her to go with (I had a bad experience semi-vacationing with my MIL (not the stepmom) and have no desire to do that again). How does DH (or DW - just realized I don't know your gender) feel about the entire situation?
You don't want to start out your vacation annoyed at someone about the timing. Try this. Make the decision to just keep your current schedule, give it a little time to sink in. See how you feel. Do the same with moving the trip to April. Which feels better?
If you do decide on April, check out things like hotel prices before giving her a yes answer. Make sure it's still within your budget.
Good luck with this decision; I know it's a difficult call.
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