I entirely agree with Prudie and LW, except: "certainly not as important as her soon-to-be grandchild".

LW's mother has no responsibilities for this child, and SHE gets to choose how important the child is to her. It'd be great if she has room in her heart to treat this child like a grandchild, but that is not an obligation.

I know my hackles would be up to the fucking MOON if, say, my mother remarried and was all "congrats, this is your soon-to-be sibling, this is now defined as MORE IMPORTANT than other priorities in your life that may conflict, deal with it."