well, I think this is not a 'reason' like 'thak gayay hain shaadiyan attend kar kar kay'... I am sure there are some other reasons which he knows only, and as being a good wife you must find out the reason....
Youngest brother always gets more previlleged than any other brothers and no attending his marraige doesnt justify anything.... I am sure you will make him attend his marriage..... warna mujhay bata daina, unn ki haal waghaira ki decoration ka 'contract' main lay loon ga on my own as a gift of marraige