The misplaced loyalty thing, man -- I'm sorry you and SD are dealing with this. My mom, for whatever reason, has never trusted me and has always been suspicious of me regardless of my behavior. This constant criticism and suspicion led me down some dark roads in my teenage years and early adulthood. She sided with my rapist, and down the road, with my DS14's dad, who was abusive to me for many years and had me in treatment for PTSD, and she maintains a relationship with him and his family to this day despite knowing her meddling has caused hundreds of problems for me and DS14. She is also obsessed with my STBXAH and is always trying to discern how he "really" is because she doesn't trust me and my thoughts on it. She knows no boundaries and throws fits if you tell her no. My mom is a classic co-dependent who I suspect also has some narcissistic tendencies -- regardless, it defines how I understand and deal with her.
However, I'm an adult and I get to choose whether I have a relationship with her today and how deep it is. But nobody was there to shield me from her constant gaslighting and unfounded mistrust as a child, and it had profound negative effects on me.
You are right to limit her contact with the in-laws. They have the right to have a relationship with their ex-DIL if they absolutely have to have one (ugh), but fortunately you have the right to limit their contact with their granddaughter. The way you talk about it -- are they contacting her through texting and social media? You can block them now through her Facebook accounts and parental settings through your wireless company, and insist that their contact with her (if any) go through you BECAUSE of their outrageous behavior. She is a minor child and they're trying to catch her in their web of codependency and denial.