As much as we would sometimes like to pretend otherwise, it is a fact that there are many people who never get along with their in-laws. No matter how much effort there is on either or both sides, sometimes personalities do not mesh. And there is little that can be done about it if this happens on a basic level. As much as you want your new bride (or groom) and your parents or siblings to be the best of friends, for them to see in each other what you see in them, sometimes there can be instinctive bad feeling between the two sides.
There is no magic wand you can wave and persuade two people, or two groups of people, who have mutual antipathy to start liking one another. Certainly, people can change their opinions on others over time, but trying to force it is not the answer. The only likely outcome of this is that people will react more angrily and potentially even violently towards individuals for whom they have an instinctive dislike so when deciding on seating plans for a wedding, it is wise to think ahead and work out where potential flashpoints may arise.
In addition, it is wise to take any of the more combustible elements on either side to one side and point out that this is your wedding day. If for no other reason, they really ought to be prepared to lay down their grudges for you, for this one day. That really is not too much to ask.