he asked me today if he wanted to go back to poland to live if i could come with him
To be honest i think he is thinking of returning home, if that is the case then it is up to you whether you want to go with him or not. To me you would be crazy to go with him to Poland. You don't speak the language, your relationship isn't going great guns at the moment.
Before you do anything you need to ask him those important vital questions. Or maybe you should be asking yourself a few questions, however its not my place to say. All i believe and have to offer i said yesterday. The longer you leave asking him the questions you want the answers to the less likely you will ever have a good relationship with this guy.
Plus just think, if he goes home to Poland the situation with this girl/sister will be like it was when she came to England, apart from the fact it will be constant and not one that disappears .
WHAT I WOULD DO: Leave him while i still could
WHAT YOU DO: is up to you but don't leave it to late either to ask him the questions or leave him. You don't want to commit moving to Poland and then have the Sh*t hit the fan out there, not only would you be punished psychologically but also in the pocket!!!
So please please please make a decision either way soon before its to late and your down Sh*t creek without a paddle.
T