Is my deceased loved one’s will or trust the last word on the distribution or can I dispute it?

No, if you have reason to believe that your deceased loved one would have intended a different distribution, there are multiple grounds to challenge that distribution.  For example, your loved one may have been the victim of undue influence, fraud, elder abuse; your loved one may have been suffering from some mental impairment, (e.g., dementia) when the subject document was created.