my neice's father is single and works as an RN in California. I feel that we have a stable environment to offer her. she is 9 years old and in the 3rd grade. Would her father's health insurance still cover her if she lives in Pennsylvania and he lives and works in California?
You have two separate issues.
The niece could live with you without you being the guardian. You would need to check with your school district to see what, if any, requirements they would have. Many insist on at least a limited guardianship, in order to prevent people from selecting school districts for athletic programs, for example. Many areas have "schools of choice" programs available, however. A practical concern would be how you can legally handle decisions for the niece, if her father is not available, (such as getting medical treatment for her). Many states allow parents to delegate parental authority using a power of attorney form or other similar authorization. These provisions are sometimes for a limited time period. (In Michigan, it is 6 months, which can be renewed, as often as needed).
You should check with the school system first and then determine whether either Pennsylvania or California allow parental delegation of authority, and which provision is most beneficial in your situation.