If there was blood on the mother bird - maybe her underbelly, then she was probably attacked by a stronger bird - otherwise yes sadly it sounds like she was poisoned and she's either regurgitated it into her babies or they've died of starvation. A neighbour who doesn't like birds could have laid out some poisoned bread or seed.

You would have to look all over her dead body to get some idea.

Last year - a young magpie visitor had a hairband stuck around it's mouth and neck. It couldn't eat. This year there's another young magpie with half a spool of dental floss wrapped around it's leg. It's leg is pinned to its chest. Fortunately he can fly and eat, but it won't be long before the floss catches on a branch and he dies a long painful death.

Human carelessness causes suffering and death to a lot of wildlife.