And tbh he's not to great with MIL herself either 
He is 15mo and he will come round with MIL when she is in but will cry after me and dh if we are leaving him with her . He does settle quickly though . But if he sees her dp he just burst into tears , sobs and all ! 
Honestly , the problem is ds just doesn't know them very well at all .
I had hoped she would realise and make an effort with him but she is too busy with her other DGS and her DPs DGD . On the rare occasions I will ask if she can look after ds , she always always brings her other DGS . Now , I try to just tell myself she is just encouraging a cousin relationship , however , she babysits my nephew a few times a week , plus every Saturday afternoon . She never takes my ds down on a Saturday afternoon for them to play together .
Anyway my point is that they barely have a relationship at all , my nephew and her DPs DGD stay overnight with them all the time , and my dcs will never get the chance of that kind of relationship because ds is scared of her dp and no one seems to think it's a problem 