In the picture you posted, there is only a sister on one side of the joint. The thru bolts in that picture are probably used because the sister isn't very big/not a lot of glueing surface. Because of space limitations from the gas tank location, I used only one sister on my stringers. I made the sister board extend aprox 12" to either side of the joint and used thickened epoxy glue. I used some counter sunk screws to clamp the glue and I just glassed over them. I ground off the excess glue so that my polyester resin would stick properly. Including screws or bolts into the joint won't hurt anything, but aren't necessary if you have a big enough gluing surface and use epoxy glue.