I am sorry you are so upset. From your post I don't feel as you stated that you have gotten past your sibling rivalry. You still seem jealous.
If your Mom wants to stay there and help her, that is her choice. Raising two children is difficult. Parents are needed for help. As far as her marriage being strange that's not your place to comment, marriage is private between two.
If you are an accountant "with a designation" maybe your time would be better spent helping her reach that goal. Be supportive. The fact that you put that "designation" in the post when you are discussing a family problem seems to me you think it makes you better because you learned some things from a book, and it does not.
Maybe if you get rid of your attitude you will have some family harmony?
Just the way one person reads the post.