Jealous Nephew (Long)

I'm a FTM with a 6 month old girl. My SIL has a 19 month old boy.

We have very different ideas when it comes to parenting. Before I had my LO I didn't think much of it because it's her child and she can parent how she wants. However, since having my LO I've been struggling with how to handle my nephew around my baby.

A little info... My SIL and her husband do minimal intervention with their son. He essentially does what he wants and they don't really do anything. They may tell him nicely not to do something and he'll just ignore them and continue on, and they don't do anything further.  However, my inlaws(the grandparents) and my husband are more firm with him and when they tell him no he usually listens. So it's not like he isn't capable of listening, he's a very smart boy.

Here's the thing, he is very jealous of my LO (and apparently babies in general). If his parents or the grandparents are holding her he will cry, throw a tantrum and try to crawl into their lap, essentially trying to push my LO out of the way. He's also very rough with his toys around her and we're constantly telling him to be gentle. The other day my SIL told my husband and I that he hit a baby at playgroup because she was holding it. She didn't seem bothered by it at all. My issue is that when he gets like this no one seems to try to deal with the behaviour.

For example, my MIL will be holding my LO in the recliner and my nephew will come crying over to her and try to scramble into her lap. Instead of telling him no, she tries to pick him up and put him in her lap too. She's a very short/stout lady and my nephew is big for his age. The entire time he's squirming in her lap and pushing against my LO, and it makes me really uncomfortable because I don't want him to hurt her.

To make the whole thing worse my MIL and SIL are always trying to push them together. My SIL tried to force them to sit together to take a photo and my LO ended up getting kicked in the head over it. My husband has repeatedly told them to stop forcing them together, but they always say they aren't even though it's very evident that they are.

I'm not sure what to do. I try to be firm with my nephew, but I don't want to overstep my bounds and upset my SIL. I don't have a great relationship with my SIL or my MIL, so I think that has been really hindering my willingness to be assertive.

Any suggestions on how to handle this?