Straighten and strengthen this joint?
That railing is probably fastened to posts on both to the left and right of your picture. 


One scheme to shore up the existing railing would be to come up from underneath the existing railing with a new wood piece that is the same dimensions as the existing one and screw it up into the existing rail. If the new piece is straight it will push up the sagging joint. Make sure to run the new piece from post to post for best looks and maximal strength.
Another idea would be to cut out the existing rail back to the center of each post on the left and right. Then lay in a new rail piece that is spliced in over the existing posts.
The last and likely not the most feasible idea would be to install an additional post just under the joint. Make the added post high enough that it pushes the sagging joint up to the position it should be in.


I have to say that whoever installed that railing in the first place with a splice like that mid span between supporting posts did not do a quality job. A splice should normally always be over a supporting post of be part of a double or triple beam made up of pieces of lumber with all splices offsetted a good distance from each other.