Joseph Duggar Gets Supportive Email From Joy-Anna's Husband After Arrest

Joseph Duggar's arrest on child molestation allegations prompted an email from his brother-in-law Austin Forsyth ... with Austin offering to help Joseph's wife, Kendra Duggar, get through things.

In new legal docs, obtained by TMZ, there's an email exchange between Joseph and Austin, who is married to Joseph's sister, "19 Kids and Counting" star Joy-Anna.

Austin emailed Joseph on March 25, while Joseph was behind bars at the Washington County Detention Center in Arkansas following allegations he molested a 9-year-old girl in Florida.

In the email, Austin tells Joseph ... "I want you to know that I love you! Jesus loves you, and continues to draw you closer to Him. I don't want to discuss your issues over these communications. You need to only be talking to your attorney. Please know that God works despite the sin of man for his good. I am praising God that we started getting closer to you guys within the last year and a half. He knew that Kendra was going to need help. We will, and are helping her!"

Joseph replies ... "than yall so much for your support of kendra! it means the world to have family like you! im soo greatful for all the people that hae jumped right in and been there to support here. love yall so much!"