awww bless your heart.
What I think is - you are grieving here, asking 'Why why why???' This is good. But I don't know if there will be an answer. It just is how it is. Maybe your Mum had a hard labour, maybe her rel with your dad wasn't good at that time, maybe she found it hard becoming a mother, who knows.
Me I am the middle, elder sister younger brother. My mother only wanted boys. Disappointed with my sister, her first - terribly disappointed with me. Didn't bother with me at all. Abused me all thro my childhood, hitting, drowning, banging head, spitting, smashing plates round face etc, shouting I hate you, i wish you'd never been born and so on.
I too dont see them.
I think you should carry on asking why, and coming up with no rational answer, and I used to lie on the floor and sob, and then feel much better, and in time I got sick of asking why, cos there is no answer, and then what the other said would happen, happened - I moved on. It is still there but it doesnt hurt as much. i guess I gave it the headspace it needed and now it doesnt often need it.
I think tho that you might want to stop asking your sisters to understand. They don't sound like they want to. Plus, it sets you up as the one at the bottom of the love pile again, begging for love. F that.
I think, grieve it out, and limit their access to you, and work on loving yourself because you are worth it. You always were. Is this the lesson?