I have a single entry type C Schengen visa which has 10 days left to be valid.
My point of entry would be Poland but I have relatives in Sweden, would it be a problem at the borders since only 10 days are only left on the visa?
Not a problem as long as you exit the Schengen area within the time of your visa you can stay until the last day. They might question a bit at the immigration when will you exit the country if you met one. But going to Sweden from Poland you won't get checked anyway because there is no border checkpoint.