It depends how do you define "sin". Yes when I left I did attend to birthday parties, I have attend to other Christians Churches as well, been to Buddhist Temple, so from WT perspective view it is sin.
But I kept my moral standards, since I believe what Jesus said about fornication, lying etc. I didn't try any drugs, or any other bad behavior
On the other hand I have some wordly friends who become and are JW's that saved their lives being in the Org. They used be heavy drug users, immoral life, (they would have died either from drugs or sexually transmitted decease)
A common mistake that some ex-Jw's are doing is trying to do what WT prohibits. This is not the way to see things, I have followed a mainstream Christian faith, (Babylon the Great) but still I kept my high standards, And one thing WT likes to point out against ex-Jw's is that they left a moral way of living and become evil and immoral.