ok jplumbley, I'm going to have a bit of trouble answering your question because I don't have my modding tools until I get home.
But firstly, its not advisable to modify any existing prop exemplars (that are already in a prop pack), especially if they have been publicly distributed. For example, taking one of Cycledoggs tree props and just adding the extra prop family iid rep, then saving cycledogg's pack with the changes you made. If you do this, and distribute your version as well, the two exemplar files will have the same iids as each other, but have different properties. Therefore if both are present on a players machine they will cause a conflict.
If you wish to use an existing prop exemplar, eg from cycledogg's prop pack, you must first make a copy of that exemplar. (Open the prop pack, find the exemplar, right click on it, copy file then start a new .dat file and paste the exemplar in your new file). Then, very importantly, and immediately, change the exemplar group and instance to a new, unique value (right click on your new exemplar, then click New Group and Instance). Save your new .dat with a new name.
Now on your new prop exemplar in your new dat file, you can delete the first rep of the Building/Prop Family property (which would be cycledogg's id) and replace it with your family id.
Now you have two separate prop exemplars. Both will still work properly, ie cycledogg's will still be in the family he put it in, and your new one will be in its new family, and no-one will have any conflicts.
I'll try to re-write this more coherently, and with pictures once I get home.
However I think your new family is not showing because its exemplar still has the same iid as cycledogg's exemplar, therefore the LE (and the game) is only reading one file and not the other.
Also, make sure that for anyone else's props that you use, that you have their permission to modify them, before you distribute your modified versions.