Yeah, the showering the grandson thing is tricky, because it could either be innocent or predatory and its hard to tell which at first glance. For example, when I was 10 my Granny used to hang out with me when I had a bath - we lived together and I liked it when she read me stories in the bath. She would sometimes comb my hair for me as well, which I loved - she was very old and sick at this point, so it was a way we could bond and hang out. I never felt unsafe, violated or upset - I loved hanging out with her, and loved lying in the bath while she read me books. Those are some really happy memories for me. Our family is very chill with nudity, so when my older sister and I visit home, if we're having a good convo our family will continue to chat when someone is in the bath.
That being said, incest/sexual abuse isn't just an ACTION, it could also be described as a DYNAMIC, and in this context the intention of the FIL is really important. Is he in the bathroom because he's chatting with the kid (e.g. the kid asked him to bring a bottle of shampoo)? Is this normal within the dynamic of the family? Do the parents of the 10 year old feel about it? Most importantly, how does the 10 year old feel about it? Does the child feel uncomfortable/violated by the experience? Kids tend to be able to pick up on the 'vibe' on an interaction pretty well, and if they feel uncomfortable that's a sign that something is up. For example, I have never once felt uncomfortable undressing/being naked in front of my mum and sister, because its normal within our family culture and I know they aren't looking at me in a weird way. However, I did used to feel weird getting undressed in front of the person who eventually sexually assaulted me before they even did anything (in changing rooms). I couldn't figure out why I felt weird about it as a child, but now I know - it's because I was picking up on their predatory intention toward me.
Obviously, the comfort of a child is not the only barometer to figure out if the FIL is predatory. Grooming can mean that children beg to stay with their abuser, or children can simply be innocently unaware of predatory intentions toward them. However, it's a pretty good place to start off with - if the child doesn't want to be left alone with an adult, even one who seems innocent/nice, that should be questioned and investigated.