hi, Where does she sleep? Is it possible to put her into your room/ nursery and keep you other little one occupied downstairs/ somewhere else?It may not be your son as such, don't forget babies have a 45 min sleep cycle, she may be in a lighter sleep so anything would wake her.
This happened during the summer holidays for me as Jake has two older sisters, who have lots of noisy friends!
I sometimes found myself having to do e.a.s....then some quiet activity for 1/2 hr is, then....s again, but always woke him for next feed.
This will happen for one reason or another alot, don't get hung up on it.
Liz x