Do I need to take legal steps to take over my sister's finances?

The one thing you do need is your sister's consent. Then she could appoint you her attorney-in-fact in a durable power of attorney for finances, and you would be legally empowered to handle her finances for her. You may be able to accomplish this without a lawyer by purchasing some reputable forms or software. Or you could draft the document and hire a lawyer simply to look it over, which may help save a bundle in legal fees.

If it is just a matter of paying bills and nothing more complicated than that, your sister could instead simply put you on her bank accounts as a joint owner and you would then be authorized to write checks to cover her expenses. You would also automatically get any remaining funds at her death, so you would have to be sure that would be her wish.

However, if your sister is not able to give consent to these arrangements due to serious illness or disability, then your best legal option would be to secure a conservatorship or guardianship for her, which would give you the same power to handle money matters on your sister's behalf. For more information, go the website of your local probate court. Many locales have good self-help centers, enabling many people to proceed without hiring a lawyer.