I feel like I am in an impossible situation, sometimes I just want to leave them all and start a new life - another I feel so sorry for my partner as, if I were to do that, she would have nobody as her mother is dead, she has no contact with her one brother back in Poland and her daughter treats her like dirt much of the time.
The question I have is why should the relationship between your partner and you end because of two other people. If your partner is in the age range of 50-60, she will have seen and experienced a lot in Poland. In Poland the family is everything and your partner will provide for her daughter until the day she dies, and the daughter will take constantly, she believes she is doing nothing wrong,it is just the Polish way. The only person to change the situation is you, you could find another house/flat that is only big enough for you and your partner and assist the couple in finding their own place, even if you have to be the co-signee, you may not be aware but it is not uncommon in Poland, for two families to live together in just a three room flat of 45 mtrs, just remember you can choose your friends,but you can't choose your family and you are family now and that comes with responsibility in Poland.