J.S.
Hmmm, I don't know how you can fix the gate, or what suggestions you could make to have it safer, but I do think you should just talk with your sister and admit that you're worried. Don't go over the top and make it seem like you think her home is unsafe, just focus on the fact that this particular thing (the kids being allowed in and out of the gate on their own) is not something that your kids are used to, and you're worried that your two year old, in particular, won't know what to do if she finds herself able to wander off on her own.
I think if your sister knows that you're anxious about it she'll likely take some precautions on her own and/or just pay extra special attention to ensure your kids are safe AND that you can enjoy your time away without being worried. Oh, and if your older child is mature enough, you could ask him/her to pay special attention to their little sister while visiting their aunt.
No matter what, enjoy your time away and check in with your kids and sister each day if you want to. There's nothing wrong with a quick phone call to put your mind at ease.