I'm so sorry you have had to endure this situation for so many years.
I don't know why a parent will favor one child over another but unfortunately it happens.
Perhaps because you by nature might be stoic and he might think you don't need help?
Aquarius by nature can be self-assured and appear to others to need no help. They
also tend to not ask for help or complain. Most Leos I know can be very charming, and they tend to
get what they want with their natural charm.
I'm generalizing here because I haven't seen your chart or your sister's chart.
I'm sorry your mother passed, I know it is very hurtful to lose your mom.
I've been liking a product called, Master Key.
http://www.luckymojo.com/masterkey.html
It is great for many things but I've found it also brings favor to you.
If you want to read about the oils go to:
http://www.luckymojo.com/oils.html
The I can you can't products are also excellent.
Just remember you're dealing with a Leo,they can be tough to compete with.
You almost have to think counter intuitively on this situation.
Another good product that comes to my mind about your Father is an influence honey jar.
http://www.luckymojo.com/influence.html
http://www.luckymojo.com/honeyjar.html
Your Father is in his 70s correct? Now that your mother is gone he may need more attention.
Can you bring him something to eat. No matter a man's age you can always tame him with good food.

The Lucky Mojo sells cook books with some sneaky tricks in them.
http://www.luckymojo.mojocatbooks.html
Think of how your sister acts around your Father, he's responding to something.
You may need to liven up more, be more charming to him, act like you need his advice or help.
I'm just thinking out loud.
I'm sure some other people here will have some good suggestions too.
Big hugs to you!