Sharing Our Innermost Thoughts

in my family there are 3 members .mom,dad and me.
my mom is patient of brain tumour and high blood pressure .
my dad is well and good as of now but he always talks negative about everything .
He is a big negative person in his life .he has habit of scolding everyone for those small tasks upon which normal people just panic a little and then let it go but he himself has habit of making every small issue a big hit.
whenever he becomes angry he use sentences like …tumlog to mujhe maar daloge.
now the problem is my mother listens to him everytime,she cries after listening but never reply to him. this is from very long time . i tried to make dad realise …by showing anger at first .
Result : he didn’t changed a bit and even blamed mother again for my behaviour .
i hoped that she will understand me but no !! she always said not to talk with him or not to make him realise that he is doing wrong
OK then i keep quiet …i thought maybe i am wrong …i should keep quiet and then everything will be ok because at the end i want my mom’s health on top priority.
for some days …result was kind of good …atleast a peacefull environment at home
but let me tell you after some more days he again started keeping quiet and angry at the same time .then one morning he was again frustating over mom …that day her B.p. rise too high .but instead of saying a word against him or even making him realise politely she said its ok let it be .HOW CAN I LET IT AS IT IS???( its been 15 days he is not talking to anyone in a usual manner but you know what ? he is taking his meals properly,he is using phone all the day …maybe wwe or news etc .he is even talking wonderfully to one of his friend on call.and you know the sad part is …all these gestures are being realised by me and not my mother.) I LOVE MY MOTHER SO MUCH
pleaseeeeee suggest me some ways