i have a GRP system and a "pipe to pipe" branch (it was an unequal tee but now client changed in pipe to pipe).

when it was an unequal tee, i set sif according to EN ISO 14692-3 (2002), par. D.2.3

Now in nomore an unequal tee.

How can i calculate the sif for "pipe to pipe" GRP branch?

If i don't insert sif, caesar will use "sif=1" but due to wrapping of pipes i imagine sif will be greater than 1.

You may need to reach out to the piping manufacturer for guidance, which they may be prepared to not deliver to you an answer.

If such is the case, it should be on you to "strongly recommend" to not circumvent engineered components as it encumbers your ability to provide realistic results, and that you shall not be held liable for deviations you cannot control.

With that said, if this user has a habit of doing this elsewhere, I would try to compare similar service and material for justification, and make an attempt to match that setup.