Why on earth do you think that by having both of you in college, each of you would automatically get enough aid to lower your costs to the point that your family is only paying for one? Has your sister already received her aid package for next year based on the information that you will be in college somewhere? Have you already received a bunch of admissions offers and aid packages indicating that you would only need to pay about half of the costs?
Take the offer from UMD, and don’t look back. Best case: your sister gets more aid next year because your family has two in college. Worst case: your sister’s aid doesn’t change - but even then your family only has to pay for one kid.