Am I sinning if I date and marry someone one who got divorced for unscriptural reasons? The responsibility is on me to try to determine this? This can be very difficult or almost impossible when the divorce happened years ago and you only hear parts of one side of the story. You may not ever know for sure.
This is a question you will want to address to your pastor. While I can try to provide responses to many different kinds of questions, yours is one that requires a level of pastoral counseling. It will be helpful for your pastor to provide him with much more information than I have here. I encourage you to contact your pastor.