I don't know if you've already done this, but I would try to obtain
Samuel BROD's death certificate. Hopefully it will list his parents
given names. If they match the parents names on Bertha's death certificate,
you will be all-but-certain Samuel and Bertha were siblings (since you
also have other evidence of this). If you can nail down that they were
siblings, then maybe researching Samuel in more detail will turn up
useful info for you about the family back in Europe (e.g. his US
draft registrations or naturalization records, if you haven't already
I would also ask myself how I know for sure that the 1932 passenger
manifest (showing Bertha going back to Europe) is actually the right Bertha.
Maybe it is indeed the right Bertha, but I would want to re-check exactly
how I know this. For me, it sometimes helps to write things out. I may
have missed something in your post, or you may have already done these things.