Weddingbee boards

My FI and I both have some family that we are not expecially close to.  He has some half-siblings from his father's first marriage, but is only close to one of them.  The other two have never been on good terms with his mother, something that has become increasingly difficult on my FI and his brother since the passing of thier father (nearly 8 years ago).  Recently they have found out of the engagement and have made comments about plans to attend.  My FI thinks this will be quite awkward and wavers on whether or not to actually invite them.

I have family from my mother's first marriage (that ended when I was three) that have tried to maintain contact through the years, but whom I've never been that close to. They have respected my mother's custody and wishes over the years, but seeing them reminds my mother of a difficult time in her life. I mentioned inviting them to the wedding and while I know this is not easy for her, she made no indication that I should do anything different for her sake. 

 My question is this: Assuming they (his sisters and my grandparents, aunt,uncle, and cousins) are invited and accept the invitation, Where do we seat them?!  They do not have the same relationship with us that the others seated in the "family" section all have, but technically they are family.  Would it be inappropriate to seat them behind those we consider our "real" family?  Should we simply allow them to find a seat amongst other guests?  We don't want to hurt anyone's feelings, on either side, but we don't know what the most appropriate act would be in a situation like this.  I'd appreciate any input on this topic!