If I remember correctly you have up to a year to send a present. And they have 3 months to send a Thank you and both should sign the Thank you.
The only dress information that is ever mentioned is if it is 'Black Tie" otherwise you don't say anything. But no one wears white except the bride.
And if your older children do not answer the invite; you do not bring your younger four children to a formal sit-down dinner and give less than it would cost to take them to McDonalds! (That is the truth of a family wedding.)