General

A taser.

Well since you didn't give any info about your sister

Give her a small bottle of some nice perfume. shampoo/body wash/lotion sets are also good gifts for girls.

Then again, I know nothing about your sister.

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Get her some pads.

@readers thanks, that's a pretty good answer.

@icyflamez999: I don't personally know your sister, only you know and can make the decision.

Since you're talking about indecisiveness, if you have multiple things that are good options, just select a random one. Flip a coin or roll a dice if you have to.

my indecisiveness will take over, narrow things down for me friend :')

Whatever you think your sister will like.