This is a complete moment from the Twilight Zone and I would appreciate some feedback because it is "off the scale" twisted and weird.

I check my emails while having a cup of coffee in the am and DH does the same as we sit in bed.  This morning I opened an email from an old, mutual friend of BM's and DH's. I have consulted for this individual's business because DH asked me to assist them. My relationship is on an acquaintance level with this person and I have not done any business with this individual for a couple of years (BM sabotaged the business relationship).  

Background: According to DH, BM failed at intimacy in their marriage from early on. One of the reasons/excuses she gave him for withholding intimacy was that she had incestual relations with a family member.  DH thinks it is one of her brothers.  BM never really gave him the whole truth and failed to seek therapy. However there are many witnesses who have seen BM get frisky with any male in sight when she has a few drinks. 

The email was directed to BM from this person. I am going to assume this individual accidentally added me to the email and didn't know it.  OR...did she know it? 

The subject line on the email is: Is your daughter from M*** ?  (M would be the brother)

This was the body of content: (seems very angry and caustic and btw this individual used to be in a relationship with that particular brother of BM's.  He just packed up and left her one day...no notice, nothing.)

Maybe THIS is why she is SO GAY ?? !  Or maybe HE had sex with her like he did you ...& THAT is why she is GAY ! 

Have a nice afterlife correcting your evil.

WTF?  So I showed it to DH and we sat there staring at each other for a minute trying to figure out what this meant.  We think this person is inferring SD. DH and I have also wondered why SD is completely relationship-avoidant. SD has not contacted DH in almost 6 months as she is angry with him for calling her out on her passive-aggressive, entitled behavior.  I am happily disengaged.

In previous talks, DH has confided, "I often wondered if SD was really mine...but she has the family short toe so she must be." I found this odd. If you are in a solid marriage you don't "wonder about someone's fidelity."  However, BM cheated on him and he knew it albeit he lived in denial.  SD looks nothing like DH and is a carbon copy of BM.

History:  Prior to his marriage with BM, DH married a woman who claimed to be pregnant with his child.  It turned out that the child was the doctor's who she worked for. The woman's father was the one who spilled the beans as the doctor wanted a paternity test. Knowing it was the other man's child, DH still tried to work it out with this woman because he loved her. The marriage was anulled. DH moved to another state and a few years later BM had a whoopsie while they were dating. They were using birth control because DH was now extremely paranoid. DH refused to marry her until he was assured SS28 was his. Let's just say it wasn't a marriage made in love. DH wanted to be a stand up guy because the baby proved to be his. SD24 came along when the marriage was already having alcoholism and infidelity issues. 

DH's face was pale. I can only imagine what he may be thinking.  I didn't think when I showed him the email and now I wish I didn't.  

The wheels are spinning.  This is right out of some twisted novel.  My concern is DH's well-being.