I had this with my mum a few years ago. She left me, DB and Dsis with our grandparents when we were very young and she just sort of popped in and out of our lives whenever she felt like it.
When I was about 15 she turned up with a baby in tow and it turned out, that was my younger brother! I was furious that she could have another child knowing how little she did for us (her other three) and a year later we found out she was pregnant with my youngest sister...
It hurt. A lot. That my mum could play happy families with two new children but still treat my brother, sister and I with such indifference. I cared for my half siblings so I promised myself that I’d rather she make her best efforts for them, than hope she failed out of spite. This was for their sake of course.
Now I’m 30 and have children of my own she and I have a better relationship- but it still took me instigating contact for a long time...
I’m sorry you’re having this situation thrust upon you OP, it opened up so many wounds for me when I experienced it. Especially the feeling of being the ‘unwanted’ child.
Do you and your Dad have a good enough relationship to raise these matters with him?
When I decided to invite my mum back into my life I had a very serious conversation with her about how it was for my DD and DS, that if she didn’t want to make the effort I wouldn’t chase her anymore (this was after a while of my instigating contact) and since then she’s made a fair effort.
I know my situation isn’t yours, but when I told her I wasn’t going to chase her for contact anymore I accepted that I didn’t need her in my life before my DC were born, so if she wasn’t involved afterwards I didn’t stand to lose much (how can you miss something you never had before?) 