Reinforcing a pair of Bent Double 11.5 Liftarms under Rotational Tension
The replacement part is available in trans clear plastic, I would suggest using them to either replace the existing beams or be connected to either side. It will be a bit cheaper if you decide to use white beams instead.
The poor quality of the clone brand pieces is one thing, but I think you might want to consider tackling the issue of weight. If you are already happy with making modifications to the structure, consider removing or substituting some of the pieces in the floating castle.

Perhaps in addition to any modifications I would also suggest using a clear fast-setting super glue.
I use Loctite Clear Super Glue to adhere tips to my pool cue and it can last well until the tip has worn down. It is best to roughen up the surface a bit before applying the glue so that it adheres better.
Make sure you only glue the pieces and not the actual connections if you want to be able to dissemble the model. Give the glue plenty of time to set without putting any tension or compression on the joint.
The result will be nearly invisible, and may could even be stronger than the original beams depending on the quality of plastic used.