I have been cheated by my husband

I got married to a person who was already married and surviving with 2 kids. I was kept under blind that he is single. After 2 years of our marriage now his 1st wife threatening to file against us saying that he should leave me or else she will file criminal case against me and my husband. I was completely unaware of his first marriage and as per their conversations its clearly shows that initially they both agreed with in them (without my knowledge) upon marrying me and she even getting monthly alumni from my husband. Now i am completely lost and unable to understand what to do, please help me in how to counter file her allegations as i have become a bait for their plan.