Try Tuscarora Mill in Leesburg.
I'm not sure what your taste is, but Leesburg is pretty small and has some really charming restaurants. See the following link for a list:
If your sister wants to do an outdoor ceremony, you could try getting a permit for a ceremony at Ida Lee Park. (http://www.idalee.org/parks/default.aspx) It's a lovely little park.
I would recommend staying away from the wineries and B&Bs if you're on a budget. Those places are shockingly expensive!!!
If your sister's heart is not totally set on Leesburg, I would recommend the Maggiano's Italian restaurant chain. They have party rooms available in Chevy Chase, MD and the Tysons's Corner part of Fairfax County. I had my wedding there. We had about 60 guests, and they were able to accommodate enough room for our ceremony and dance floor. The price was quite reasonable, service was excellent, and the food was great.
In case you don't have a cake vendor, try Chantel's. They were very reasonable and it was the best wedding cake I've ever had!!
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