In Her Place by Edel Coffey

Who is the other woman? That's for you to decide.
Ann devoted years to her mother's care - and now she's gone, Ann feels lost.
Justin is also grieving, but his wife is still alive. Deborah is in a coma and she doesn't have long left.
When the two meet, they are instantly drawn to one another and, before long, they've fallen deeply in love.
Ann quickly moves in with Justin and his little girl, making them the perfect family. But just as Ann settles into her new life, Justin's is turned upside down. Unexpectedly, his wife has survived. Deborah is coming home.
Neither knows what to do. But one thing is Ann has earned the life Deborah left behind, and she plans to keep it . . .