Do I have the right idea regarding I 864 Affidavit of Support?

Hello everyone
I'm planning to be a joint sponsor for my son.
My daughter in law (USC, the sponsor/petitioner) is sponsoring my son (beneficiary) but her income wasn't enough as seen on their joint tax income return 2018.
He's still waiting for his EAD.
I've lived with them since October of 2018.
I've received a divorce decree this April 2019 from my 2nd wife. My son is from my 1st.
But due to a mistake with tax filing back in 2014 me and my ex have been filing separately since then.
We've stopped living together since early 2017.
We have no children and I have no dependents on my tax records.
But I've filed Tax return 2018 married but separately.

After reading the Instructions for I 864A I don't think I should file this form because I'm not my daughter in law's "spouse, parent, child, adult son or daughter, or sibling relative".
I'm also not a dependent on their joint income tax return. Am I correct regarding this? That I should file I 864 also like my DIL?
My ex also wouldn't need to file I 864A because we're divorced. My income is enough and I don't consider her a member of my household. Am I correct in assuming this? Do I need to provide proof that we're divorced though?

Also on the form I 864 regarding household size. Does my DIL put 2 (her + my son)? Since I can't be classified as "household member" based on the I 864A instructions.
On mine should say 2 as well (me + my son)?
Just asking this because we all live in the same address.

Please correct me if I'm wrong.
If you have better ideas or options please let us know.
Thank you so much.