My Husband's Relationship With His Sister...

Okay, I'll try to keep this short. I have no idea what constitutes a normal brother and sister relationship being that I do not have a brother myself. However, my husband does have a sister who is four years older than him, and from the beginning of him and I dating I found his and her relationship to seem a bit off and a little too close for comfort. I try not to be crazy and to talk myself out of thinking weird things, but recently some pictures have come to light that have made me extremely uncomfortable. I feel that they only validate my feelings about their relationship even more. I was looking through our bedside tables the other day in search of a heating pad and on my husband's side there was a photo album that I had pulled out of the drawer. I decided to look through it out of sheer curiosity and it turned out to be an album that his mother had put together with pictures of him and his family from a few years back before him and I had met. As I flipped through I was taken aback when I came across a picture of him and his sister laying next to each other on the couch, sharing a pillow and laying under the same blanket. When you think about laying next to someone on a couch, you can understand that in order to make space for two people you typically have to squeeze in pretty tight, essentially you are spooning the other person which is what it looks like they were doing in the photo. What disturbs me most was how old they are in the picture. My husband is probably 22 and his sister being four years older is probably about 26. I continue to flip through the photos. I then come across another picture of them when they were younger. My husband was probably about 13 or 14, so his sister would have been about 17 or 18. They are sharing a recliner chair and what looks to be a living room. They are both using and sharing the same blanket again, it is draped over both of their laps. She is slightly propped up because it looks like the chair was too narrow to fit both of them sitting in it side-by-side. So, she is propped up with one of her butt cheeks on my husband's lap, essentially sitting in his lap. He has a remote in his hand so I can only assume they were watching TV. I continue to flip through the photo album. The last photo I see is a group photo of my husband, his sister, their little brother, and A couple of their childhood friends. Everyone else is posed normally, except for my husband and his sister. He is sitting down on what looks to be a lawn chair and she is draped over his lap. His hand is reached around her lower back and resting on her hip and she is grasping his thumb with her hand.

I do not want to sound ignorant or psychotic by any means, but these pictures made me feel extremely uneasy and almost sick to my stomach. From what I was taught, this is not normal behavior between two adult siblings. Am I crazy? I want to discuss this with my husband because it's eating me up inside but I feel like it will only offend him. We have had lots of issues about this in the past and throughout our whole relationship. We have thought about his sister and their level of closeness and we have almost split on multiple occasions over this issue. Any advice?