As step parents have no "legal" rights to the children, they do have obligations as a parent. A step parent can seek medical care for step children, and sign things that "parents" normally sign. This happens mostly when the step children live with the step parent; but can happen at any time. (visiting etc) Since being a step parent means being a parent, they have the obligation to the step kids, but not the "rights". So in other words, step parents do all the work and act as a parent, but if something should happen they have no legal standing. So yes, most places will allow step parents to sign documents, and seek medical help. If they have a problem doing so (which usually they don't) all they have to do is get written permission from one of the bio parents to allow them that "luxury".
------------------
*There is no love, like the unconditional love of a Mother*
-I am not an Attorney, any advice given is strictly advice-