Are N's incestuous? Mother jealous of son's GF?

Hi everyone, new to this forum and I'm so glad that I found it. I've known about NPD since psych class in high school, but maintained in contact with some seriously malignant narcs for years before I even realized it.

I was surrounded by abusers for most of my life, starting with my childhood ex best friend. She had somehow convinced me that it would be a good idea to date her brother (my now ex N) to get his "evil" ex gf jealous.

There was always this nagging feeling in the back of my mind that both his mom and sister had this pathological jealousy towards me as soon as we got together. Examples include:

-His mom inviting his ex over for valentines day dinner and maintaining a positive relationship with her.

-His mom covering up for him after he had gotten violent with me, claiming that I "provoked" him.

-His sister asking for intimate details of how he was in bed.

-His mom inviting and allowing a random girl (he cheated on me with) to stay at their family home while visiting from out of town.

-His mom "unfriending" me on facebook after he and I got into an argument.

-Him sharing a bed with his mom while on vacation

-He and his mother going out to watch a very sexual film at the movies together.

-His sister throwing verbal dirt in my face by saying, "my brother doesn't care about you, he has a real girlfriend and you're just a booty call."

-His sister constantly talking about his ex and personal details about her life. She also told me that she practiced witchcraft and put a curse on me.

-His sister orchestrating fights and drama between us.

I can go on and on about this but it would take me all day to write it down. I'm just wondering why his sister would minimize our relationship so much if she was the one hooked us up in the beginning? There's clearly some incestuous vibes going on and when I brought this to my ex N's attention, he claimed that he didn't know what the word or concept of incest meant (yea, right). Has anyone ever experienced something where they felt as though the N and his/her family depended on each other for emotional support, almost sexual? Pathological jealousy from their family members?

I'm currently 10 months NC with the N's family, which I'm very happy about because I no longer feel like my emotions are being controlled by these sick people. I officially broke up with the N one year ago, but I'm only one week NC due to his successful hoovering attempts. Any advice or just some real, straight forward talk is more than welcome.