<human>: Condense the text into a few key points

My fiancée and I are starting to plan a wedding for this Fall.  Both of our families live out east (in the US), we live out west, and we're planning on getting married in the south, so basically everyone is going to have to travel there.

There are no issues with my family, but she has 3 siblings.  Her older brother and sister are each married and they, and their spouses, are both great.  Her younger brother is OK, but his girlfriend is a mess and nobody in the family likes her.  She has cheated on the younger brother, and it was discovered that their child is not even the younger brother's child.  She uses drugs, lies constantly about even the smallest things, steals, is a leech on the family in general, and always finds a way to insult my fiancée whenever she can -- and my fiancée is quite sensitive to it.  Most of the family, including my fiancée and I, hate being around her and think she would cause some scene at the wedding if she was there.

I am 100% against having her at our wedding, and my fiancée doesn't really want her there either, but there's obviously going to be an issue if we invite the rest of the family and not her.  She definitely wants her brother there, but it'd be hard to invite him and not her.

How can we deal with the issue of wanting the rest of my fiancée's family there with the exception of the younger brother's girlfriend?  Her idea was to tell the rest of her family not to give any monetary support to the her younger brother/his girlfriend in the hopes that they could not both afford to travel to the wedding, but I don't think this would work that well as it's not THAT expensive to get there and they'd have plenty of time to save for it.
<bot>: My fiancée and I don't want to invite a trashy member of her family to the wedding but it be very awkward to do so.  How can we solve this issue?