I think it is important to remember that racism is unacceptable in modern life. But we all have to understand that it is not the term 'Polish', 'Black', 'Ginger' which makes a statement racist. It is the tone, manner and context which it is used.
If by using the word Polish, you are using it to suggest a 'lesser person' (derogatory statement), then you are racist. If it was sincerely in a discriptive nature, then you are not. However by using the words Polish & Scum in the same sentence and Polish at a time of high emotion you lead people to assume it is meant as a derogatory statement.
So if I say 'Mr X is a Black Man', I am not racist, just using a commonly used description of a Negro Male. If I say 'Mr X is a Black B'stard' then i'm not inherently racist as the 'black' in the statement could have been meant as a descriptive term, however by using the word in this context I would find it hard to justify it's use in such away. It is my responsibility to make sure that I am not misinterpreted and categorized as Racist.
The difference between the statement Ginger B'stard and Black B'stard is simple, the Black people of the world have in history been treated as lesser persons, due to the colour of their skin, as such it is more commonly beleived that the 'black' in a derogatory statement is a sign that you are suggesting that the person is a lesser person due to their skin tone. As far as I know Ginger people haven't been persecuted in such a way.
So as a conclusion do I think your post was racist?, I hope not. Do I think you are racist, I hope not?. From not knowing you and reading your post, could I be drawn to the conclusion you are a racist/xenaphobic? Yes I could. Does it matter what I think of you? Not a F'ing jot.
I hope you Mother in Law is getting over her nightmare ordeal.