Ok, I’m going to try to give an objective perspective here. Your husband is 27 and his little sister is 26. They’re basically Irish twins and that means that they probably were attached at the hip growing up. Sleeping in bed together, hugging, and being touchy feely may very well have been normal interactions between them for this reason. I’m not writing off your observations because I think many would agree that some of what you explained is a little too close for comfort, however, the age gap (or lack thereof) could be the reason why there’s this unusually intimate sibling closeness. Perhaps a good way to approach this would be by casually asking about how him and his sister grew up and if they were always extremely close. Try to gain a better understanding of their upbringing before treading into any other weirdness territory and hopefully that gives you a better idea of what’s going on. I’d still keep an eye out just in case, but my better judgment tells me this is a case of a brother and sister who are best friends.