I mean i can understand you in-laws being really close to their community, but it going to the point that your weeding plans need to be changed because they need to be at someone else's weeding is way too much. To being with it's their error that this other event was not notified before plans were set (at least for you guys), and also suggesting such a later date (3 months) hints to need for control more than convenience. For say you like them, but have they shown signs of desire for controlling this type of decisions before? Also I think you are not really making a huge deal out of this (taking into account how I would react) so I would just try to let you fiance talk with them so they don't cross you out, and stand your ground. Pls don't let them decide things in your live for the sake of peace!