Is it permissible for the uncle to sleep in the same house as his niece and nephew (both 20+ years old) while their parents are away? if not, is there a solution?
An uncle may not sleep in the same house at night with only his neice and nephew. This is Yichud and it is also a situation called libo gas bo. The options I can think of are to have another boy sleep over with them. Another option however but not easy to do, is to give a key to a neighbor who actually will come in a few times during the night. This is not very practical, becuae the nieghbor has to be a person that will actually do it, which will generate the needed fear) and not merely say that he will come in, becaue we all know that he most probably isn’t, and it isn’t generating a correct “mirsus”.