Should I talk honestly to my brother about his girlfriend, risking our own relationship?

My older brother (in his 60s) started dating a lady with Asperger's. She is hyper-sexual and flirty. She does not understand boundaries. They've been dating for a few months, and she's talking marriage. Another sibling and his wife have met her, and they have said she is never invited back. My brother always picks similar ladies. Do we speak honestly and risk losing a relationship?