Yes, it is a sin. Per Islam and Quran Golden rules, there is no compulsion in religion (see related question below on Quran Golden Rules). You should have full free will to believe in the religion that chose. However, you should know also that free choice is responsibility. You hold responsible of your choice in front of God, the Creator, on the Last Day of Judgment. The other point, if you chose to convert into Islam then your choice should be based on true belief in Islam and in Islam principles and not solely based on making your marriage legible (see related question below on Islam main principles).