No i don't think it's appropriate. If she's quite young how is she to know what's going on, she may mistake the noise for a threat to their safety. If she's older what's being done to help her cope with being exposed to such occasions - i presume this isn't the first time she's dealt with this? What's the deal with her being left to look after a younger sister? If something happens and she needs her mother is she just supposed to walk in on them and ask?

At what time is this happening? Does the older girl also have to make her sister dinner and/or put her to bed? Age is an important issue here - if the eldest is an adult then really it's her decision on how to deal with the issue and you'll just have to try and support her as best you can but i agree that in either case it's not an ideal situation. I hope everything resolves for the best.