Cat stress

Question
Barb-
I hope you can help my situation.  I have two cats, brother and sister from the same litter, 10 months old. The female cat has started throwing up and suffering with brief bouts of diarrhea once or twice a week for the last month (happening at roughly the same time).  .  We took her to the vets who said she was healthy, but as a precaution had us give her and her brother a deworming dose (they both had it as kittens) and recommended we change their food to adult cat food (we use Hill's Science Diet).
The last two events seem to have been a result of the male chasing his sister through the house at night.  She became agitated, jumping up on our bed and running across it, multiple times last night.  She seemed to be reacting solely to her brother's presence. We then heard what sounded like her passing gas, so I immediately thought she was going to let lose her bowels and put her in the bathroom with the litter box and shut the door (she did go and vomit later).  I left her there 2 hours overnight until we woke up this morning.  She has also been jumping up on us (not unusual) to be petted, but she has her mouth open slightly (unusual).  Before the past few weeks, the two got on fine, with the occasional feline "argument." They still feed and play together.
Our local vet could only suggest getting cat food that is easy on her digestion.  I'm wondering if this is a behavioral issue and we should take steps to correct it. Should we separate the two at night? We can do this, providing each with food, water, and a litter box.  Should we discourage the male from chasing the female, or provide a "haven" for the now skittish female to hide like a box with a hole cut out set in a corner?  
Sorry for the long winded e-mail, and I hope you can help.  Any ideas would be appreciated!
Thanks,
Rob


Answer
Hi Rob,

I really want to write a complete answer to this.  I just haven't had much time.  You know how the season keeps you busy and all.  I have a few ideas for you that might help.  Can you please write me at my regular email?  That might help, then I can answer you when I have a few minutes instead of trying to get into this every time.  My address is:  redheadf99@yahoo.com.  (don't forget the "f".  Many people do)  Barb