Helping My Mom

My mom found out she had a half sister, but the sister died before my mom even knew about her. We found out that she had a child two days before she passed away, and that child was put up for adoption. We are helping each other try to find family members (I am helping her find her half niece, she is helping me find my biological family members, I am just taking a step back from my search for my biological because I'm so stressed out.) This child would have been born December 4(?) 1978 in Pasadena, CA to a Teresa Ann Williams (Williams was her maiden name.) We cannot locate anyone who is alive for any help. Her husband at the time has passed away, her parents have passed away. It was a closed adoption but we know about this child because Teresa passed away due to complications at birth, and her gravestone that we found says "Mother" We are assuming that the father but the baby girl up for adoption due to Teresa passing away right after, not knowing what to do. Things like that. My mom has done DNA testing from Ancestry and 23andMe. What else could we do to find this child? My mom would love to meet her and tell her about her moms info, info about Teresa/my moms dad. Have her meet people she is related too. What can we do?