I can't describe how my heart dropped into my stomach when I read the last paragraph. You were a fantastic uncle to your nephew, and the guilt should not sit with you (though that is easier said than done). In life, I can only imagine they were grateful for the relationship they had with you, even if it was from a distance after that incident.

Consider therapy, if this is weighing heavily on you. Consider encouraging anyone else you think might be affected by this, to consider therapy. If appropriate, perhaps consider seeing if you can close the gap with your niece, but I understand you may not want to rock the boat when things are unstable.

Don't forget to love eachother. Don't forget to love yourself.