Poland. Custody Of 11 Yoear Old

On mobile, stressed and english is my second language.

I am an adult sister od the child in question and this doesn't afect me. I call that man a dad but he is a step dad i met when i was like 10.

The lawyerish advice is free for those who cant afford so it is not best and i want to know what kind of evidence might be usefull - ie. Xray of my cracked skull with some evidence that it happened when i was child and wasnt taken care of - is it usefull in that case or not

What is the best course of action for father (not married but he is on birth certificate) who has limited contact. Mother forbids him from entering the house (she is the owner), talks about him like he is the monster etc and he is blocked on all devices and if not she is the one replying (i wont belive in 11 year old sayin stuff like - our meeting at 2pm today is canceled - which is very formal in our language and this child regressed in reading since father left due to skipping school and being an iphone/xbox/tv/attention span of goldfish child ).

My mother is mentally sick, lives in her own world and dismisses anyone who says she needs help. Mentally, emotionaly, financialy abusive and also add to this medical neglience.

My father picked up some bad traits from her but unlike her he works on himself, apologised and seeks psychiatric help - from what i know he suffers from anxiety due to her.