He's never wearing pants!

My nephew is 3 and whenever we go to my SIL house to visit, he never has pants on. Like he's just butt naked. 
And he hates it! 
There's been numerous times he's brought me or my mum or my partner his trousers and asks us to put them on for him. 
When we look after him he hates having to go to the toilet, he often dribbles on himself and so his pants need changing but he always kicks off about it until he's certain another pair is getting put on him. 
I don't know how to bring it up with my SIL, I've spoken to my brother and he knows how his son hates it but he works away at least 4 days a week so isn't always there to make sure he's covered up. I personally think it's really weird that she always has him half naked. She smacks him bum playfully and always comments "how cute his little butt is" 😕
My mum thinks she does it because he keeps having accidents and so it's easier for Her to keep him naked instead of having to change him all the time, but I think she should dress him when she knows she's having company or if it's a suprise visit, dress him right away
What can I do to make her realise it's wrong that's she's doing this? Luckily he hates it, but he could be one of those kids that goes to nursery and starts exposing himself because he thinks it's normal to have his bits out all the time. 
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Unfortunately those thoughts have crossed my mind also, especially with the way she talks about his "cute little butt" or calls him her "sexy little man" 
But he's such a happy little boy ALL the time that I find it hard to believe. I really don't know what to do. 
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I'm not jumping to any conclusions.
I should also have mentioned he is not being potty trained. He is out of nappies day and night and uses his potty and the big toilet all by himself. She started naked potty training. But he's learned now. And has been for almost a year 
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yes she is a lazy parent, it was my mum who suggested that maybe it's just easier for her to not put his pants back on if he is having accidents a lot. BUT he's usually pretty good with NOT having accidents so I don't think it's that. He does have to take meds to help him poop so he does sometimes follow through a little when he pumps. But that can't be helped. 
I just want to bring it up with her that he's clearly uncomfortable and unhappy being naked but she is the type of person who would think I'm having a go at her and probably tell me to mind my own and raise mine my way and leave her to hers