I would call up your local DMV or appropriate state department and ask them what to do. Most states allow you to pay a small fee and show an out of state status and they will issue you a temporary paper plate for the vehicle to be relocated.
Whether you're insuring your car or RV, temporary insurance is not really an option — most policies are for 6-12 months. You can typically cancel at any time but be aware of any fees for canceling early.
If your sister is the policyholder, you will have to have her call her insurance company to access the policy information. Since the car is registered in your name, the insurance either needs to be in your name or you need to be listed on her policy as an additional interest.
You can get insurance in your name but your girlfriend will need to be listed on the policy since the car is in her name and she will be driving the vehicle. If you have any further questions, don't hesitate to ask. For more information, see our additional resources below.
You need to have the title transferred to you and register it in your name. Here is some information related to that — it will all be done via the TX DMV. There are some forms you will need to fill out.