if you have legal papers as durable power of attorney that your parents appointed you, can a sibling be the parent's health care proxy?

No. A health proxy is the same thing as someone that is Medical Power of Atty. It cannot be changed just like that and needs a signed document by the current medical power of atty revoking their privileges.

Sorry I disagree, a person may change their advocate at any time for any reason as long as they have the cognitive capacity to do so. In some places they are required to inform their former POA in writing, but in others that is not a requirement (although it makes good sense).
Often families appoint different people for health and finance, reasoning they don't want to overburden one, sometimes people choose two or more who can act "jointly and severally" (either/or), and hopefully everyone has named a secondary who can step in if the primary can not perform their duties for any reason.