I have relatives like this too. Its very thoughtful of you to think ahead and try to figure all these little details.... I would simply say that you might want to split the difference and, while trying to provide as much help as possible, just make sure that she doesnt see you going to such lengths that she feels her fears are justified.

You can reassure her that everyone on her flight will be going to immigration together, at the same time. Many of those people from her flt will speak her language, not to mention that part of the CBP job is to frequently deal with passengers that dont speak English... although this situation is not normal for her, its very normal for them.

Also, Im assuming that if she is coming from SAP, she speaks Spanish? If this is the case, Im pretty sure that in SAP & IAH she'll have no problem finding many people she can communicate with if she has a question.