She's being inappropriate to try to parent over you and you need to set very strong boundaries with her. She's overbearing and thinks she knows better than you, their mom, and this will get worse if you don't stop it. She is using your kids as standins because she's alienated everyone else in her life.
Janet, please don't tell my kids what to do in their own home.
Janet, I'm the parent and I will direct my kids if I see the need, not you.
Janet, don't ask my kids to do things without checking with me first.
Janet, what we eat is not your concern.
Janet, you're overstepping.
If she tries to kick back, just tell her no, she wasn't there when you lost your husband and their dad and she is a friend, not a part of your family.
What someone said ^ about this could be tricky with teens, yes. She says things you know aren't true, like how she helped bring them up. She's trying to force closeness with them.
I'd back off from her.