I have read both of your threads from start to finish this evening OP and my heart absolutely goes out to you.
You are the daughter of a narcissistic Mother, every thread you wrote about your Mother made it clearer and clearer. Your Mother does not care about you, she cares only about herself, about retaining control and going on as things were before.
She thinks everything is fine now that she has talked about her feelings of guilt towards your sister. Of course it’s all fine to her now, as she is a narcissist, she has no empathy or care for your feelings. You are her scapegoat, she has admitted that and your sister is her golden child.
I advise looking into narcissistic Mothers- this website is excellent:
www.daughtersofnarcissisticmothers.com/
The creator of the website has also written several books
www.amazon.co.uk/Youre-Not-Crazy-Understanding-Narccissistic-ebook/dp/B00EF5L2VW/ref=sr_1_fkmr1_2?keywords=Daughters+f+narcissistic+mothers&tag=mumsnetforum-21&ie=UTF8&qid=1547867301&sr=8-2-fkmr1
I also advise reading the Susan Forward books - Toxic Parents and Mothers who can’t love:
www.amazon.co.uk/Parents-Overcoming-Hurtful-Legacy-Reclaiming/dp/0553814826/ref=sr_1_1?s=books&keywords=toxic+parents&tag=mumsnetforum-21&ie=UTF8&qid=1547867392&sr=1-1
www.amazon.co.uk/Mothers-Who-Cant-Love-Daughters/dp/006220436X/ref=sr_1_4?s=books&keywords=toxic+parents&tag=mumsnetforum-21&ie=UTF8&qid=1547867392&sr=1-4
Another good book is this
www.amazon.co.uk/Will-Ever-Good-Enough-Narcissistic/dp/1439129436/ref=sr_1_6?s=books&keywords=toxic+parents&tag=mumsnetforum-21&ie=UTF8&qid=1547867392&sr=1-6
Whilst the books will no doubt be helpful, you have deep seated hurt to deal with, maternal abandonment at such a crucial time of development, will have affected you deeply! Not to mention your Mother never repairing the maternal bond, choosing instead to lavish her maternal attentions on your sister!
Your Mothers admission of not loving you from birth and feeling no guilt towards you upon her return, will have torn off the band aid on your heart and made vulnerable the tiny baby who was never loved, never valued enough for her Mother to even feel guilty for leaving.
I would seek counselling for JUST YOU to explore that, certainly well before considering any family counselling. For once, someone needs to put YOU first, to care about YOUR pain, YOUR hurt and YOUR emotions and feelings, throughout both your childhood and the last few months.
Between the books and counselling, hopefully you will be able to create healthy boundaries and find a way to move forward.