I need some misconceptions straightened out about Jewish mourning. I've heard some say the death of someone in the immediate family of a betrothed must delay the wedding for anywhere between 30 days to an entire year out of respect for the dead. I've heard others say that a wedding completely overrides mourning unconditionally. I've also heard that a person due to die just before their child's wedding can order their child to perform the wedding on schedule, overriding any prescribed interruption, since a commandment to obey parent's final wish to see his child wed should override a duty the respect nominally due to parent. Which is right?
Nitei Gavriel Hilchos Aveilos 2 Chapter 15 & 16 discusses this question. From what I see the only time a wedding would get delayed is when the bridegroom or bride are themselves in formal mourning. It would definitely be delayed. In some instances until after the Shiva and in others until after the Sheloshim. Parents and grandparents who are in mourning may participate completely in a child's or grandchild's wedding. Regarding siblings, he differentiates between unmarried siblings who may participate and married siblings who can participate if they are serving, however not for the complete wedding.
The Jewish approach is to always give precedence to happy events over sad ones, as the happy event are with the eyes to the future, and with an understanding that although our beloved one is not among us (even though his soul is eternal) his continuance is expressed by his descendant that is building his home. On the other hand, a delay must not be rejected right away, since the tension and the bad atmosphere might harm the happiness of the event. In conclusion you must try to make the event on its date but it is definitely legitimate to delay it.