Option 1

Sit down with your cousin privately and explain your financial concerns honestly, suggesting alternative, more affordable housing options or other ways you could help that don't involve co-signing.

Option 2

Tell other family members about your cousin's request and ask them to intervene and convince your cousin not to pursue such an expensive apartment.

Option 3

Avoid your cousin's calls and messages until they give up on the idea, hoping they will find another solution without your involvement.