I don't think he will get it for a while. You will probably just have to keep repeating yourself when it comes up. And it might get a little embarassing to them at some point but they will just have to understand that he is a little guy and doesn't mean anything disrespectful.
I have 6 year old twin daughters and two sons that are 19 & 15. My oldest son had a girlfriend for a while that would come over and visit with him. The girls loved this girl and thought she was the best.
My son broke up with her and starting dating someone else. But before they came over, I tried to explain to my girls that Babu (their name for him) wasn't with #1 anymore and that they couldn't ask about her when #2 was around. They couldn't understand why #1 wasn't going to come around anymore or why they couldn't talk about her.
Girl #2 came over with Babu and right away the girls ask about #1. We had warned brother about this and told him to warn #2 about this too, because the girls were not trying to hurt anyone or be mean - they were just acting their age.
And they did ask about #1 when #2 came over. It was a little embarassing for the grownups in the room but #2 was very gracious about it.
I would explain to your sister and brother-in-law that your son is just not getting it and probably won't for a while and that they just have to be understanding about it.
I hope this helped.
