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I went through your query in detail and can really understand the distress you must be in. I am really sorry to hear about your mother. You have really been through a lot throughout your life and at this young age have to manage through lots of difficulties.
I can understand that you and your brother don't like the person your sister is in relationship with. But that should not be nay reason to break relationship with your sister. If she is happy with a particular person, you can just accept it as a fact. Moreover, you yourself accept that the person in question has definitely helped you all in times of need.
You have mentioned that you have been working. So, you can make an independent decision about your life too. Going to Australia to your sister is not the only option in front of you. You have your whole life ahead. You can very well work on improving your career, later on select a life partner and settle down at a place of your choice.
More importantly, I feel that currently you are having symptoms suggestive of a depressive . I would recommend that you seek a psychiatric consultation for the same and initiate treatment. Medications like escitalopram or sertraline
would be really beneficial and effective helping you out of current symptoms and will make you feel better. You will also be able to take better decision with a better frame of mind.
I do hope I was able to answer your query. Please let me know if you have any further queries. i will be glad to offer help.
Dr. Sunil Gupta