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Could you help me with these sentences grammatically?

1 She's the one who chooses who she wants to see at her wedding.
2 What could she have been thinking?
3 We'll have even more (of) a reason to disinvite if he does that.

THank you
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1. She's the one who chooses whom she wants to see at her wedding. [What you've written is just fine in most contexts, but, if the speaker is a English aristocrat, or if you're speaking to a grammarian, be sure to choose whom.]

2. OK ['What was she thinking?' is a more common way of saying this.]

3. We'll have even more of a reason to uninvite him if he does that. [Disinvite is an obsolete word. I added him to prevent a miscue. The sentence you wrote is grammatical.]
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Thank you for your help,

regarding Whom and Who

Could you tell me what you think of this?

The only person who/whom has bought me a gift whom I didn't buy a gift for is her.
How would you say this please?

Thank you
The only person who has bought me a gift but who I didn't buy a gift for is her.

In both instances, who is a standing in for the subject of the sentence. Therefore, whom is incorrect.
Could it not be

"but whom I didn't buy a gift for is her"

Isn't it

"the brother whom I dislike the most is him"?

Thank you Futurist
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As it turns out, I did make one mistake in that sentence: 'her' should have been 'she', i.e.

The only person who has bought me a gift but who I didn't buy a gift for is she.

The second 'who' is the object of the preposition 'for', but it is also the subject of the verb 'is'. 'Whom' should not be used as the subject of any finite verb.

'The brother whom I dislike the most is him' is also incorrect. It should read:

The brother who I dislike the most is he.

'Who' is the object of the verb dislike, but it is also the subject of the verb 'is'.

The 'she' in the first sentence and the 'he' in the second sentence are both nominative predicates, i.e. they are equivalent to the subject of the sentence. She is the only person. He is the brother.