I would like to get more involved in my mother's health care. My sister has POA and she tells me mom is ok. Is it worth me getting involved?

I believe it depends on what degree of involvement you are seeking. Having Durable Power of Attorney is a lot of work. It can also be pretty stressful. It sounds like you are expecting to have disagreements with your sister before you even become involved. Your sister might not be hiding anything from you, even if it looks like she is. Had you been previously involved with Mom, helping with her care and decisions that had to be made on her behalf? Did you and your sister work together on this in a cooperative way? If you’ve suddenly come on “the scene” and are demanding that your sister now start disclosing what is going on, she may feel resentful. If she does share Mom’s health diagnoses with you, are you prepared to work with your sister to make things easier for her? Talk to your sister in a non-confrontational way and assure her that you, like her, have Mom’s best interests at heart.

Who told you about the back problem? Presumably not your sister then?

Why would discussing your mother's health care necessarily mean fighting with your sister?

I think it would be nice that all children be made aware of a parents health whether your POA or not, unless a parent says they only want certain children to know.

if she’s in a hospital, show up for the doctor’s rounds. If you live too far away, ask your sister to call you and put you on speakerphone for the rounds.

If if she’s in a facility go visit. It takes courage, but you will be glad you did.