Difficult Situation

I have a family situation that I'm not sure how to deal with anymore. 

We have an in-law suite in our daylight basement that my brother in law and sister in law rent. They have a son who is 3 months younger than our son (2.5 years old). Both boys have always lived together and have been raised together.  Our kids have VERY different personalities but I'm so tired of biting my tongue when I notice bad behavior from my nephew fostered by his mom (side note: my kid is NOT perfect nor is our parenting but we try to alter our parenting to address each phase as it comes up). She is still nursing. Normally, I wouldn't think twice about this as I nursed my son until after he was 2. BUT, he completely relies on boob for everything. Sleep, comfort, "I just stubbed my toe and need affection", my cousin stole a toy and I want it back, etc. Like, he cries out to nurse at every little thing. He also has no bedtime routine at all except to watch his iPad until he gets tired and then fall asleep on the boob. Both BIL and SIL claim that it's impossible to get him down in his own bed ever. He is latched throughout the night and doesn't take naps unless she is taking one with him and he can nurse. At this point, I feel like it's doing more harm than good. It's IMPOSSIBLE for anyone else to take care of him because he Loses. His.  Shit. He can't be in a room where she isn't without throwing a tantrum. Whenever they take an overnight trip away, our nephew sleeps through the night and he's asleep within 10 minutes. Our son is the same and we worked hard to get that routine established with him - it was not easy or fun and required lots of trial and error on our part to get to where we are now. We DON'T claim to know all the answers but we do know what works for us and how we've gotten our nephew to sleep (in his own bed) through the night. Both BIL and SIL claim to want to know how we manage it but make no actual effort to replicate the process. If he starts to throw a she just nurses instead. 

Perhaps it's my own pregnancy hormones driving this but I'm about to throw the biggest tantrum of all. I'm so sick of hearing this kid cry because of something his parents have catered to. And when I say cry, I mean, the world is ending, wails, ear shattering screams, etc. It's awful. My husband and I watched our nephew for a few nights so that my SIL could get away and wean him. Literally, 5 minutes after she walked in the door she was nursing him. What was the point in leaving if you don't intend on actually weaning?!  We just went through hell with this kid for 3 days to help with the weaning process and she's broken down and nursing again!  I just broke down in tears because I simply can't sit by and watch this any longer. My nephew is awful to be around because he is so attached and reliant upon her. I'm just at wits end and don't know how to deal anymore. Am I letting my hormones get the best of me?  Should I just continue to tune it out?  What would you do??  Sorry for the rambling vent. I just honestly don't know who to ask any longer and my husband is getting sick of hearing me complain under my breath.