Lab in first heat, what can I expect from her brother?

Question
Our 11 month old chocolate lab is spotting (first day is today), which i am predicting her first heat. What can I expect from her brother?
She and her brother have been raised together since bringing them home, and I have had to go back to work full time 4 months ago. They are at home in their big outdoor play area unsupervised for approximately 9 hours a day.  They do not like to be separated, but I am worried about her getting pregnant by him. (And yes we plan on having her spayed, but thought she was too young at 5 months and with going back to work full time, time has passed so quickly).  We both work full time away from the home now ( I was working from home up until a few months ago), leaving them inside is out of the question as they are still puppies, and love to see what they can do "new" around the house (especially her!)
I am hoping we will get through this without her getting pregnant and we will have her spayed as soon as she is able after being in heat, but in the mean time is there something I can do to help eliminate the "temptation" of wanting to mount her or be mounted while this is going on?  My husband does not want to have the male fixed.
As like the birth of a child, we are learning things everyday raising these two.  Any profound knowledge or insight would be helpful.

Answer
You must separate them.  Otherwise, they will mate again and again through her entire fertile period ensuring she becomes pregnant by him.  The second and third weeks after the beginning of the discharge are when the most risk of accidental breeding is. Don't take this lightly. She will do anything to get loose and get to her brother or any other  male. Once loose, an accidental breeding and an unwanted litter is far from the worst that can happen. Such breedings can also result in fatal infections. She can get killed when she dashes heedlessly across the road in front of a car. Other dogs or coyotes may kill her.

She will have to be kept in a secure area, I have heard of of males jumping a 6' fence to get to a female.  It might be best to keep her inside the house in a crate the entire 2 weeks.  An outdoor kennel with a roof and well secured to the ground would protect her.  Unfortunately another male could jump your fence and get into a fight with your male.  One good option would be to find a kennel that would accept her and leave her there the 2 weeks.  The breeder you got them from might be willing to keep her.  

Don't expect normal behavior from her, him, or any other males in the neighborhood.  Even if you haven't seen any other males before, they may come for miles.