She is very self absorbed and cannot be wrong.
Have no doubt that she will feel the very wronged victim in all of this.
But why wouldn't she?
This is how she has been reared.
To be the most important one, mothers favourite.
You are deeply conditioned to expect little for yourself.
You have children, be careful of passing down to them that it is ok to be treated poorly by people who should care for you.
Ask yourself honestly, is this what you want for your children in their future friendships/ relationships?
All giving in one direction?
I don't think you do.
Our children watch, see, hear, absorb.
On the sister thing, another friend had a sister that was similarly self absorbed and was very focused on herself and her life with her husband, travelling and living a great life. She would visit briefly, regale them of her fab life, with little interest in anyone else.
At 39 they wanted a family and when she found out she was pregnant they decided to move home after living away for 18 years.
Everyone was very happy for her, but when the baby arrived they really expected her two sisters with late teenage children to be available to give them time off, babysitting etc.
She expected her two nieces to be keen on babysitting too.
Her husband was no better, expecting his side to step in so they could continue to have a great social life.
They were both deluded.
Neither of them had EVER babysat their nieces and nephews, having never been even slightly interested in them.
Their siblings were all past the baby stage and had zero interest beyond an initial cooing.
They were all busy with work and their own lives and neither were interested in slotting in weekend babysitting now that they were finally enjoying some freedom themselves.
Their teenage children all had part time jobs and had no interest whatsoever in giving up their weekends to babysit, and my friend refused when her sister had the nerve to demand she insist.
Firm words were had, and a clearing of the air.
They loved her, were delighted she had her baby, would definitely be there in an emergency, BUT, they all were long past that stage and had zero interest revisiting it.
They certainly weren't going to be telling their 19 year olds how they should spend their Saturday nights.
Her sister admitted that she had thought they all would be "thrilled" to help her.
She was wrong.
Many women having raised their own children, have little interest in babies.
I'm definitely this person as is my friend.
Her sister now has a very busy toddler and herself and her husband have adapted somewhat, but very quickly decided they were one and done.
They go on a lot about how permanently exhausted they are, so stopping at one was wise.
Focus on your friends and these relationships and hopefully space from your sister will help you set better boundaries and expectations of how you should be treated.
Long term this will serve you well.