Syd35 If I were you I would talk to someone about this, someone who can be of help.
Do adoption services have anything useful to say on this, I wonder if anyone else can advise.
I am not sure what would be best. The future for this little boy is of paramount importance but I am not sure how seeing you would affect him. I would imagine it would help him but I just don't know.
I think if there is someone you can discuss this with, then it would be helpful to do that now.
Talking to someone outside the family would not commit you to doing anything or not doing anything but might just make you more aware of the possibilities.
I think I can totally understand how your sister would feel (I mean as much as anyone can understand and put themselves in another's shes!). I mean if my child had been taken away, for whatever reason, and my sis were to see her, I think (hope) I would feel pleased and at least it was a chance to know how the child was etc BUT I can really see how she would feel upset by the fact you saw him and not her.
I think it is best for you to do whatever feels right for you. You seem like a really caring person. If you will feel happier and better knowing you have tried to see him then that is what I would do. I am sure that you are aware that you may (or may not) upset your sister etc but if your concern is to see your nephew and if this is something that is allowed and is considered helpful for him by those who are caring for him then I think it is totally reasonable for you to look into this.
Will you feel better if you do that? Are you doing it because of love and concern for him and also because it will be good for you to see him? If so, and assuming that the visit goes smoothly and does not upset him then I can't see a problem with it.
BUT I don't know - I am NOT an adopter so I have no idea of all the ins and outs of the law about this but I do think it is reasonable for you to ask and find out. Please do keep in touch with us if it helps you.
Thinking of you.