I'm sure she'll get pissed in general/and or grossed out, as I would if my ex husband slept with my former mother in law, I think it will lead to a lot of fun conversations between her and Lucas.
Her Ex (Rafe) could become her other Ex'es (Lucas) new dad. Would that make Rafe her ex father in law as well as her ex husband? Will's and Allie's new grandpa is also their stepfather (former).
And Sami's new grandchild is also Rafe's nephew, who could one day be his stepson's son, while it's actually his uncle....