Adult son has divided family

[DLB1960]

Here is my story.
I have two grown sons. They each have two children. My oldest son (will refer to as "A") has a boy age 6 and girl age 3. My youngest son (will refer to as B) has two boys ages 7 and 5. My sons have polar opposite parenting styles - one has a lot of structure and boundaries (many good) while the other is very free-spirited with few boundaries. Family gatherings are stressful because of these differences. My son's live on the west coast. I live in Iowa and visit for the summers.
Almost a year ago during my visit, my grandson (son of B) showed his penis, with his brother at side, to his cousin (son of A) in a tent at his birthday party. Son B and his wife immediately apologized, acknowledged it was inappropriate and said they would address it. Son A and his wife deemed this "not normal" and that the child and parents needed professional help. Lots of hurtful words were exchanged from son A.
As I consulted both sons, while respecting their viewpoints, they insisted I had to choose sides. I communicated that I would not. Threats were made to me that I had to choose sides, or they would have no choice but to not have contact with me. They each wanted me to make a stand against the other.
In October, a child protection service claim was made against son B and family. The report I am told was very specific and seemed obvious it came from son A and/or his wife. Details including the penis exposure, an incident at a past family vacation, my grandson's speech (deemed autistic in nature), etc. against son B and family. Following the delivery of the report and investigation, son B's wife texted the boy's family members making them aware of report. This "accusation" infuriated the other family who claimed no responsibility and chastised them for blaming them.
I had a great visit in February with both sons. The topic was not avoided but not discussed. Shortly after my visit Son A/wife discovered an Instagram post wife of Son B posted regarding their pain of a family member attacking their family in submitting a Child Protection Services claim. I “liked” the post. This infuriated Son A/wife who proclaimed that I had chosen sides. Since this “discovery” Son A has not allowed me to speak to his children and has thrown away gifts that I sent to them.
I have returned for my summer long visit to be near my sons and grandchildren. I now have a healthy relationship with son B. Son A has demanded that I make his brother apologize to him and his family for the exposure incident and has become verbally abusive to me and many family members. Son A is not allowing me to see his children. This is the hardest part.
Son A has shared that he wants resolution with his father but with me he says he hates me and has a narrow response of “you know what you have to do (make his brother apologize)” no matter what I say. There is no rationalization or calm discussions between us. I am not certain if it is Son A or his wife is driving the anger and judgmental mentality.
I have shared with Son A that he has choices; it is not my chose to not see my grandchildren and that his healing can not come from the actions of others and that it needs to come from him.
I would appreciate hearing from others that have situations that have divided families.

Our objective is to better understand the struggles our child faces and to learn the skills to improve our relationship and provide a supportive environment and also improve on our own emotional responses, attitudes and effectiveness as a family leaders

[Swimmy55]

Welcome!
Well done, you!   As painful as this is, you are right for not caving / having to choose sides. To do so would be feeding the illness.  You haven't stated which son has BPD , but taking care of yourself first is paramount when dealing with the adult BPD child. 

[DLB1960]

Son A and/or wife have not been diagnosed but in my opinion fit profile of borderline personality disorder. I am not certain if my son is driven by his wives beliefs and/or if both have characteristics as I only hear from my son. I respect and accept they may have a different opinion on what "sexual abuse" is but their inability to accept others views is painful as they isolate themselves from the family.
I have compassion for both and pray for their healing every day but I have reached a point in that I am no longer going to tolerate the verbal abuse. It is extremely hard not seeing my grandchildren (age 4 and almost 6) and worrying about them. While I believe they are cautious in what they say to the kids, they are smart and I am sure sense the tension and are confused why they are not allowed to be with family