Parental alienation seems to go hand in hand with narcissism and narcissists always choose a "golden child." If there is only one kid then there isn't a choice but PAS takes a lot of time and attention to do the manipulating so it makes sense that only children are more susceptible to it. If there's more than one sibling then usually there is one favorite that is elevated to a peer with BM and that's the one that she focuses her alienation on. When there are siblings kids are less emotionally reliant on mom so it's harder to alienate them from dad unless mom also alienates them from each other by picking an obvious favorite. 

My SD was an only child until recently and very enmeshed with mom. She's 12 and has a 2 year old half sibling at BMs house. By the way she talks about her and how she interacts with my daughter it's clear SD is more like another parent to her half sister rather than a sibling. This is the other side of alienation where as a kid gets older they become spousified. This happens more with opposite sex kids but it can happen with either if the parent elevates the child to a status of being a peer and confidant.  So it seems siblings close in age are more of a protection against aleination and enmeshment.

Often, older kids will help alienate younger kids. 

Whoops!

I can only comment on our situation.  OSD left our home when she was 13, fully supported by BM. Things had been on a downward trajectory for about year prior; before that things were pretty good (relatively as we know with SM-hood) with her and DH. But puberty hit and she started HATING DH. Basically it gradually came out that BM was likely telling "her side" of the divorce story and disagreed with DH doing any parenting at all in our home. OSD could do no wrong and had the run of BMs house. How could a teen NOT want no rules and to do what she wanted?

YSD so far has not been alienated. She's, in the end, a more independent person and makes up her own mind. However, she struggles. She does not like DH even mentioning OSD and hardly ever mentions her. DH and I have been open with YSD about how much DH misses her and how hard it is doing things w/out her. So she knows. BM will still try to get YSD on her side, I'm sure there was a lot of promotion of the high school near BM by both BM and OSD....I mean we witnessed it once and YSD talked about it with her friends - but she denied she ever talked about it at BMs house. Yeah, right. BM also supplies her with soap, toothpaste, toothbrushes, all the essentials for when she's at our home. YSD cannot figure out how to ask for things so I think she complains to BM and BM just does it. DH doesn't seem to care so I don't either. Less $ he puts out.

I think she does 'report' things to BM and OSD - but I don't care really.  And whatever DH does or doesn't do as a parent I have zero control over and I don't try. I don't parent with her at all. Never will. There are times she acts like she's completely put out to be here and does NOT enthusiastically say she likes it at dad's house or likes seeing dad (again, overheard - she's not exactly quiet when talking with friends).

Both DH and I expect that whenever school opens again we'll likely see a LOT less of her. Both because of high school activities and the fact that she just won't want to come. DH is prepared for that and tells me, "You know what, I don't even care if she does that." because he's so tired of fighting for OSD and thinks it's a losing battle in the end.

That's a great visual. I vote we add it to the STalk nomenclature.

There's just so many different flavors of dysfunction that it's hard to generalize. I knew nothing about enmeshment until another poster introduced the concept, at which point my brain lit up light a slot machine paying out.

My parents each chose one of their children to be their support in their War of the Roses. Dad and my sister were inseparable, and Mom had me, the youngest. Rather than draw us siblings closer, the dysfunction created a permanent chasm between us. There was alienation on both sides, too. Not pretty.

Then you have the enmeshment that existed between my FIL and SILs. DH was the eldest, and only boy. He was pushed aside - expectations and discipline were applied to him, while his sisters were the spoiled, undisciplined mini wives. They are all middle-aged now, and DH is still an outsider in his family of origin. The SILs don't know they were enmeshed, just think that they had the BEST DAD EVAH! DH doesn't know about enmeshment either, but the saddest part to me is that he still has the core belief that his sisters' wants are more important than his own. All of them are too close to see what their father did to them.