You seem to be asking if you can sign an affidavit of support as a "joint sponsor" and if you are a conditional, lawful permanent resident, you can.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
If you mean can you sign their I-864 Affidavit of Support, yes. Be aware, however, that by doing so, you assume some financial risk that your cousin's spouse will need government assistance when s/he comes to the U.S.
You should always consult with an experienced immigration attorney to make certain that the advice you received is appropriate for your particular immigration case.