Is it legal for an alternate POA agent to sign a parent's DNR form, when the primary POA is easily available, though not present at signing?

My healthcare POA form states that the alternate can act when the primary agent is "not willing, able or reasonably available to serve." I (the primary agent) was not present when the DNR was signed, but I was easily accessible by phone and could have gone over to sign the form. It was routine paperwork, no emergency. To me it seems the alternate agent (my sibling) overstepped their power. The nurse doing the paperwork has the POA on file, yet allowed this, perhaps out of convenience, but that doesn't mean it's right or legal. I suspect sib told the nurse I wasn't able to be there, when that was untrue. I had reason to be absent from that particular meeting yet would have joined in if I'd known such issues would arise. Nobody told me the DNR was done until I requested it. They said it was already completed. A DNR is a huge deal and I was totally left out of it- that doesn't sit right with me.

I don't want to change the DNR form-- my mom's wishes are stated there accurately. However, I'm concerned about future POA issues arising, and me getting left out of the loop again. Mom's healthcare team assures me that legally I must be included since I'm primary POA-- but then how did the DNR form get signed so easily without any attempt to include me? I'm also concerned that my sib's signature on the DNR will throw off anyone seeking immediate POA authority for urgent decisions, and they'll erroneously defer to sib. The POA form is next to the DNR form, but sib presents as the authority, and won't willingly include me or contact me (sib has serious power issues and scary unwarranted hostility towards me- sib is an absolute nightmare, to put it kindly, making it harder for me to help mom the way she's trusting me to do as her POA). Mom's healthcare team tells sib I need to be included, as a family member and as mom's appointed POA. But sib doesn't agree, since I'm not "the caregiver living with mom" (unfortunately sib is a de facto caregiver for mom part-time, not by mom's choice-- long story....).

The healthcare POA seems fairly straightforward at first-- but in my complicated family situation, it doesn't play out so simply. I need to learn how my POA rights/responsibilities can actually be exercised beneficially in my delicate family situation. In the DNR case, I don't want to make issue of the form with others or re-do it because I don't want to be petty or stir up more animosity in my sib-- yet I want to know if it went down wrongly or illegally, and how to prevent future problems. I'm trying to be wise about when to assert my POA, or when to hold back and "grin and bear it", in the best interests of my mom, family peace, fairness, and avoiding future court ordeals. The stress involved with all of this is through the roof- any suggestions are appreciated.