You will probably need to file your case in Small Claims court, in the appropriate jurisdiction, so long as your damages are under the jurisdictional limits of Small Claims. You will seek contribution (and indemnity, if applicable) from the co-tenant. Even though each of you are jointly and severally liable to the landlord under the lease, each of you can seek from the other contribution for payment of the rent. Keep an accurate accounting of your relative's missed rent payments and try to work it out informally (but in writing), outside of court, before you file a Small Claims suit. In fact, this is required in most, if not all, jurisdictions.
You can find Small Claims forms online at the link, below.
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