Hi K.,
Why is it really bothering you? I ask because there are two reasons why your sisters recent habits would bother me, if I were in your shoes.
The first being, having recently suffered such a grave loss, I would be protective of those close to me and their safety.
Secondly, I would be challenged in explaining to my child why it's OK for the aunt to live a lifestyle that doesn't support the family's. In other words, I'm not ready to answer questions of "what's all the hype in boy-girl sleepovers".
While most 8 year olds have a general understanding, the consistency in your sisters behavor, could lead to more daunting questions.
If I could make a few suggestions, it would be identify exactly what about your sisters behavor is bothering you. Then toy with possible compromises. Try to have at least 2 (if one doesn't appeal to her then you've got a backup) keeping in mind the "win-win" result. For example: sleepovers limited to Friday and Saturday nights only. You're not worrying 7 days out the week now it's only 2.
Then, set aside some private time with her and "lovingly" express your concerns. If she had no idea it was bothering you she might offer one of the compromises you had in mind and all is settled.
If a compromise is not to be found, always remember it's your right and your luxury to make your household rules and ANYONE who lives under your roof must abide by them.
In conclusion, dealing with a situation while remaining calm has much better results that letting it ride until one day, the final straw....
One more thing, I hope I didn't offend you in anyway. If I did, my sincerest apologies. My very humble, nonjudgmental suggestions were only meant to aide in a resolution.
Respectfully,
Charolette