Hi Kelli,
I swaddled my daughter until she was 6 months old. But there did come a point in time when she would wiggle her arms free and then wake herself up. So.... the trick is to get her to feel that security of swaddling and be able to fall asleep, but not wake herself up later.
What we did was go slow. I would loosely wrap her arms at first... so when she wanted them out then she could get them out. I started with just one arm. I did the loose wrap of one arm for about 2 nights. Then, I just kept that same arm out of the swaddle. I found trying to not swaddle two arms, was too much. So, after a week or so of just one arm swaddled, we went for the second one.
Once you have both arms free and she is comfortable falling asleep like that.... then you could consider a sleep sack. But, until you can get her to fall asleep well without her arms swaddled, a sleep sack isn't going to do you any good.