Contesting one of 2 beneficiaries on life insurance policy.

I am a survived sibling and primary beneficiary of my brother's life insurance policy. I was the sole beneficiary until after my brother's passing that I discovered there are now 2 beneficiaries - his domestic partner of only one year. The point is that I discovered that the change was done 9 days before my brother's death. Brother died of cancer and was on powerful pain meds like hydromorphone, had brain mets from the cancer, and he also stopped eating 14 days before passing. The domestic partner is also a male nurse. My opinion is that he had my brother signed a document creating two beneficiaries which awarded his domestic partner #2 when he was NOT of sound mind due to the aforementioned info. Is there a way to contest this? and how? Thanks to anyone for their input.