If
Shug / Shuggie works for you, then fine, but I agree with the other comments that your ds may not wish to be called that as an adult. And since your ds will only see his cousin once or twice a year, do you really think you'd even need to go there with
Shug as a nn? Have you considered just talking to your sister before the baby is born?
She may surprise you. When I was PG with my second child, I didn't really care for any male names. The only one I remotely liked was
Nicholas, but it was my sister's favorite name for a boy and at that time, she wasn't sure if she'd have any more children. So I asked her opinion about
Nicholas and she was surprisingly gracious and told me to go ahead and use it. Furthermore, if she ever had another ds, she'd use it too and we'd deal with different nns. As things turned out, I had a dd and she had a ds named
Nicholas 6.5 years later. :-)
Another story about my sister: Several years after her dd
Emily was born, her SIL asked her permission to use
Emma for her dd. Since
Emma would be living in Australia and my sister lives in the Southeastern US, my sister wholeheartedly agreed and was pleased to have been consulted. Sometimes, people can be incredibly agreeable. Your sister might be as well -- especially since
Enitan has nothing to do with
Eitan other than a few similar sounds.