This is what I have to offer you:
Firstly a very tough situation for you, you are still hurting and that's very hard for you. The way I see how you are reacting is, your nephew is family and you probably have some deep resentment there because your SIL has a baby and you lost yours - I don't think that's wrong, I think it's actually quite a natural reaction. I think deep down it makes you sad to see you MIL with her grandson and you wish she was interacting with your baby too.
Whereas the children at work, while you do get somewhat attached to them, they aren't blood, they are just other peoples children, people who aren't close and aren't part of your outside world, they are your occupation - you are paid to relate well to them.

I think it will be a healing process for you - hugs that you get through it.