How do I terminate my role as co trustee of my sister''s trust (she is 59 yrs old)? How do I give this role to my sister''s daughter (29 yrs old)?

The answer to your question is that it depends on the trust itself.  Most trusts contain provisions for successor trustees.  I suggest that you have the trust reviewed by an attorney who handles trusts to determine exactly what must be done.  It might well require only the preparation of a few documents. James B. SpinaLaw Offices of James B. Spina, Ltd.708-647-1900jbs@spinalaw.com