As much as this sucks, I don't think it's OK for a parent to block family from a 16yo.
Yes, what the in laws are doing is awful. ]
Yes, we all want to protect our kids. At 16, she is farther beyond "protecting" than I think any mom wants to admit (I sure as heck didn't).
AND - I also sense that at least some part is YOU wanting it to stop YOUR triggers. Absolutely understandable.
AND - that is about you and not your kid.
Be there to SUPPORT your kid. Teach the boundaries. Teach them the freedom we all get from being responsible for ourselves and our own behavior. Teach your DD how to calmly/gently tell their aunts/uncles/grandparents what is and is not acceptable. What respect looks like.
SHOW them how you are able to set boundaries, stick to them, and not obsess about it anymore.
IMHO, if YOU are the one to block DD from interacting with her blood relatives (awful as they may be, including to her), not only has your DD not learned for herself how to handle awful people (cuz we all know DD will be dealing with awful people in some form or another for the rest of her life), you are also opening yourself up to DD's resentment/anger/etc.
YOU get to have your boundaries WRT the in laws, including WRT your DD's interactions with in laws. That can include saying I don't want to be involved, don't tell me about it, etc ... and what message does that send to DD? Perhaps there is a way to communicate to DD your belief these folks won't treat her with decency/respect or are using her for nefarious reasons. And that it's DD's decision, tho you will be available to support her if/when the sh*t hits the fan. IMO, teens/young adults need solid support / safety net to catch them from their not so great choices. If the parent makes the choice FOR them, they aren't learning how to make solid choice on their own.
I come from a long line of family drama - that has often included kids being in the middle. That 2015 Xmas of which you speak? Yeah, I had that too... but it was me and my sibling and my mom who didn't get a gift - I was about 14/15 and my sibling was about 10. And I was the one that said I would no longer attend xmas with that part of the family. Lesson learned (albeit the hard way) that those family members did not have my best interest at heart (and worse, that neither did my own mom, but that's another story).
So - they may be hounding your DD. It may make your DD feel quite uncomfortable, conflicted, and a bunch of other feelings that aren't fun. It's a learning OPPORTUNITY for her to see how she does not like being treated as an object / disrespect to figure out how to say no - even to folks that can be charming/convincing or cause guilt (eg the "I should be nice bc they are family" dilemma). That lesson, alone, is worth GOLD in the long run, esp when it comes to romantic relationships later in life... Can you envision being able to say in 5 or 10 years - gee DD, that guy sounds a lot like aunt x or grandma Y, do you remember how you handled that? Remember how you learned to say no and to set boundaries with folks that don't treat you as you deserve? Don't treat you with respect?
If I haven't already recommended it, I found Brene Brown to have some really solid parenting advice, esp her audio series called The Power of Vulnerability - stuff I wish I'd heard when my kids were young/teens. It's 6 or 8 hours and I got it on Hoopla via my local library. Well worth every minute (and I've listened to it 2x and it's back on my list for a 3d time).