Mother's Day is the busiest week of the year for jewelry gifts, which means two things: order early, and don't buy the piece that says "Mom" on it.
She knows. What she doesn't have is a piece that fits what she wears on a Tuesday. Buy that.
What is the one mistake to avoid?
Buying something louder than she is. A statement piece is a gift to the person choosing it. If she wears one thin chain and small studs, buy one size quieter than your instinct.
Should the whole family give one piece together?
Often yes. One well-chosen piece from everyone is worth more than several small ones, and it avoids her having to wear four things at once to be polite. Put every name in the note.