I am a Muslim Brother.
Is my uncle e.g. chacha and mama mahram or non mahram to my wife?
Am I mahram or non mahram to my nephew and niece’s wife/husband? (Both perspectives from man and woman)
In the name of Allāh, the Most Gracious, the Most Merciful
As-salāmu ‘alaykum wa-raḥmatullāhi wa-barakātuh
Your uncles are non-Mahram  to your wife .
You are a non-Mahram to your nephews’ wife.
And Allah Ta‘ālā knows best
(Mufti) Bilal al-Mahmudi
Checked and Approved by,
Mufti Luqman Hansrot
 Non-Mahram (marriage-kin) is someone who you can marry.
 Surah an-Nisaa. Verse 23, 24.
قوله تعالي – واحل لكم ما وراء ذلكم
Fatawa Mahmudiyyah. Vol 11. Pg. 289 + 300