I hope against likelihood that they'll keep the poly relationship as it was - super functional and stable. The kids had different pet names for the two moms, and after Marston died, Elizabeth and Olive continued to live together with Elizabeth financially supporting the family and Olive taking care of the children. They lived together for 64 years.
This often gets downplayed as Elizabeth "accepting" another woman into their marriage, but truthfully the three were (more than would be expected considering the times) pretty equal in their relationship.
My favourite quote from this article
For the record, Marston and Olive Byrne’s son, Byrne Marston, who is an 83-year-old retired obstetrician, thinks that when Marston talked about the importance of submission, he meant it only metaphorically. “I never saw anything like that in our house,” he told me. “He didn’t tie the ladies up to the bedpost. He’d never have gotten away with it."