i never could keep them straight It means you have a hard time distinguishing the difference between two things. Example: The sisters are so similar I can't keep them straight.

when a person talks about someone "being straight", it usually means being orderly, calm, just behaving properly in general. ex. "my kids were always running around, screaming and drawing on the walls. I could never keep them straight." ex. of the opposite "those boys sure did straighten out after joining the army" hope this helps

A similar saying we have is "I can't keep track", like "Maybe she was working, or maybe it was her day off. I can never keep track." To keep something straight means keeping track of it in your mind. Straight generally means "good", or we say something went "off-track" or "off the rails" means not good. Should I use ser or estar? I never could keep it straight!

In this case it means you could never keep remember what days she works and what days she is off.