asalam alaikum wr wb,
Bro, If you are there when he hits your mum then obviously it's acceptable to defend your mum, even if that means hitting him. But I don't think it's right if you went up to him on a different occasion and bopped him one. Try to find out why your uncle is resorting to violence in the first place.
Is your mum living in Pak or just visiting? And what about other relatives? Are your grandparents still alive.? Do they know what is going on? If so what are they doing about it? Hasn't anyone intervened?
Can you ask all your family to warn off your uncle? And also tell him the Islamic view on his behavior. If he doesn't see sense, how about threatening him with legal action?
Also tell your mum, to keep out of his way and not get involved in family hoo-haas. Your mum should also make duaa for her protection and for your uncle's hidayah.
Here are some Invocations:
1) What to say if you fear people may harm you:
Allaahummak-fineehim bimma shi'ta
O Allah, suffice protect me against them however You wish.
2) Allaahumma Rabbas-samaawaatis-sab'i, wa Rabbal-Arshil-Adheem, kun lee jaaran min (here you mention the person's name), wa 'ahzaabihi min khalaa'iqika, 'an yafruta 'alayya 'adhadun minhum 'aw yatghaa, 'azza jaaruka, wa jalla thanaa'uka, wa laa 'ilaaha 'illaa 'Anta.
O Allah, Lord of the heavens, Lord of the Magnificent Throne, be for me a support against [such a such a person] and his helpers from among Your creatures, lest any of them abuse me or do me wrong. Might is Your patronage and glorious are Your praises. There is none worthy of worship but You.
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Be steadfast in the rememberence of Allah.
wa alaikum asalam wr wb.