Prepare to attend the wedding anyway, Hannelore. Your older son's attitude might change close to the date i.e. he would want his brother there. Your sons will make up, especially so when the family dynamics have always been close. With all the calmness you can muster, try to keep your communication line with him and the future DIL open. These things often get out of hand when both or all the parties are at loggerheads. Often a contrary attitude, to that which your older son would perhaps expect, might resolve things i.e. a better atmosphere than now.
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