Marriage.
For better or worse?
A karmic relationship
Usually nothing more then paying of debts.
Relationship builds
Relationship destory
***
There is no motes of love left within the
marriage; your mum is merely a companion to shoulder the entire
family burden and responsibility for your dad, where in fact, he
ought to share half of what is on your mum. Submissive wife and a
mad-raging dad, if you thought about it, the only reason why this
marriage is still on-going - it has to be you, her child.
Nothing will change.
Woman gather and listen: if you think you can change your man
through marriage, you will be wronged. A man will be the same,
before and after marriage, unless enlightenment has taken place
from within. However, usually for a man case, its difficult because
most man are not spiritually/emotionally driven.
And then again, if they are - such thingy hardly occur.
Your mum suffer much hardship; she is pretty much a strong woman.
And an emotionally strong woman would be able to get out of your
dad's clutches and survive on her own. She wouldn't. She was so
drilled psychologically that she thought she wouldn't survive...
have to be scolded... she is unlovable and all sort of nasty psychological
imprints that bounded her in this loveless marriage.

NOW, it is up to her child to tell her otherwise.
I can not only see your helplessness and agony in this post, but
also the deep love and affection for your mum. If you are
independent (ability to work), my suggestion:
I) To bring her out of this home first. She will will definitely
resist (especially if your mum is a Taurus, Scorpio, Leo,
Aquarius), but you have to keep 'brain-washing' (psychological
conditioning) her that what this marriage has imbue into her psyche
- it must now be undone and led away by her child.
II) In a new environment, everything will change eventually. She
must be cut from the karmic cord she had with your dad - meaning,
an self-impoised physical separation from your parents first.
Financially, you will be stretch to the max at first, but after
when your mum had settle emotionally in her own home, encourage and
help her to find work in so to achieve three things:
- For easing of finanicial load. (Physical)
- Clearing her mind and focus on new things. (Psychological)
- Interaction between new people; not neccessary a new mate, rather
to keep her emotions faraway from her failing marriage.
(Emotional)
III) Once her new life sinks in and when courage shine upon her to
raise the issue of divorce, this would be the final stage. Of
course, your dad WILL fight to the bone for these
three things to happen. Why would he want to lose a woman who's
nothing more than a free maid to him? He will fight and you as her
child will combat back. Resistance must happen if you want things
to change.
Everything walks a long way and love is the only way you could
bring a smile to your mum. Keep loving her, not only by action,
but also through
words and gesture like hugs. This is important for
emotional restructuring. Discard those-asian side of being
uncomfortable and embrace what love would do, like you to your
partner.
P.S: I would fight for my mum if that happens (Aries Moon in 4th
house) and she will receive million times more love than what my
dad would give. You have to share the same
sentiments. She will then evolve, for you act as her
'little husband' and would gradually gather what is neccessary to
lead a better life in the future. I wouldn't advice trying to
psycho her to divorce your dad BEFORE letting her experiencing her
new life. She will definitely resist (especially if she belongs to
the four astrological sign mentioned). Go on a reverse trip, if it
is humanly possible. 
Cheers