[Nightedge]

Could I use 'of'?

-Mary was indecisive __ telling her mother. (=not sure whether she should tell)

On GoogleBooks I got 'of' from something by Noah Webster. But 'of' seems not to pair with 'indecisive'. I'm not sure. For the above, I would think that 'in' is the only correct answer. What's a native speaker's opinion?

[Beatrix]

could it be "about"?
"She was indecisive about telling her mother."

Susan? Could you, please, help?

[susan53]

Certainly not "of" or "in". "about" would be fine. What's the context though? If it's conversation "indecisive" sounds wrong. It usually means someone who always has difficulty deciding things - ie it describes a personality trait rather than referring to a specific instances. I'd prefer :
She couldn't decide whether to tell her mother (or not).

[Nightedge]

Excellent answer. Thank you.