Can I, an 18 year old man stay at my parents house over-night if my sister is sleeping there with her two friends (parents are not there)?
If this is generally an issue, is there some way to get around it? i.e give a friend the combination to my house and tell him he is welcome to come in any time in middle of the night?
If the parents are home there is no problem, however at night even three women and one man is yichud. It is possible to give a friend a key and tell him that he is welcome to come at any time. However this will only work if this is going to cause a fear between the people in the house alone that someone may actually come in. If it is only being done as a way around yichud and they don’t mean it, then it doesn’t work.
The fact that your sister is there will not help at night, because when she is sleeping she is not a shomer.
Besides this, even if it isn’t halachically yichud, it is still not advisable, because of a lack of tznius etc.
Kiddushin 80b, E:H22-5