Oath of a minor

Q: My sister took an oath regarding a specific chair "Wallahi I won't sit on the chair". A) Her intention was that she won't sit at that moment.  B) She is 12 years old and is not Baaligh.

1) Is she allowed to sit on the chair now?

2) If she sits on it will she have to pay kaffara?

Not wishing to take one's share of the inheritance

Q: If a person has three wives, can the last wife ask her inheritance be given back to the other two wives as she does not want to be part of the the distribution although she is entitled to it by Islamic law. So the question is can she sign a document to the effect that she does not want her share and is this practise acceptable from an Islamic perspective?

Allowing someone who consumes alcohol to use one's utensils

Q:

1. If someone drinks alcohol and then uses my utensils for drinking water or uses my spoon will my utensils become napaak as his mouth will be impure?

2. If I use a dishwasher to clean impure utensils soiled by alcohol or something impure will the dishwasher be impure? Do I have to wash it?

3. Once an impure glass which was previously containing impure substance is washed can I consider it pure and use it?

4. Washing impurities in the sink makes the sink impure?