How to handle an autistic teenage boy's inappropriate behavior?

My nephew is 14 yrs old, autistic, high functioning, and very immature, comparatively. The other day we were with his grandmother (my mother in law) who is currently in an isted living facility for rehabilitation following a fall. She was in her wheelchair and he was standing behind the wheelchair, hugging her around her neck when I noticed my mother in law trying to remove his hands from her ! So I quietly removed him from her, but it happened again, in her room! She was in bed and he got into the bed to lay with her, and started . I told him it was inappropriate and made him get out of her bed. His mother, my sister in law, when confronted replied "what am I supposed to do? He doesn't understand!" Well, I don't know how much he understands or doesn't, but know this cannot continue and he MUST be taught whether it's his grandmother or a clmate, or a stranger! I'm afraid for his safety, my 77 year old mother in law's, and his future. I don't know what the correct way is to handle this. We are very involved in his life and always have been but the uality thing I just don't know how to handle when autism is involved. I have 2 adult daughters, and had no problem speaking openly to them about uality or anything else. I keep thinking, if the situation involved my daughters and a boy them, I really wouldn't care if he were autistic or not! I would be LIVID! Anyone with an autistic child have any suggestions? I know I can research online, and have done a little, but honestly, I have a 23 yr old with epilepsy and gastroparesis, as well as my husband, and have spent the last 4 years researching online. Kind of burnt out. And I really feel as though it's HIS mother's responsibility to educate and control his behavior, not ours! So, if anyone out there has any experience with this and can give me some sound advise, I'd really, really appreciate it. He is our nephew and we love him dearly, I don't think he's perverted but need the best way to stop this before it becomes tragic. Thanks to every and everyone who may be able to help.