How long to "re-acquaint" siblings after separation?

One of my foster kittens was returned earlier today... Tiger was adopted 11 days ago but they turned out to be very allergic to her and brought her back today. I still have some of her littermates I am fostering. The cats do not seem to recognize each other. Tiger has been hissing and growling at the other cats if they come near and she seems nervous and unhappy. I put her in a separate room for now. I am just wondering how long it will take her to get used to the other cats again and relax. I feel bad that she has to get used to a "new" place again too.

Awww, poor Tiger.. that's a real shame about her adoptive family! Kittens are usually easier to introduce whether they are siblings or not. Billy was about ten weeks when we brought Johnboy home and had a HUGE attitude about it. Always hissing, growling and scratching, but within 48 hours they were buds.

Hissing & growling is how she is communicating with them, so it's not necessarily a bad thing. She's very young and most likely stressed out from having to acclimate to several new environments so quickly. Unless she/one of them is being or going to get physically aggressive, I find it best to stay out of it and let them settle things themselves. They'll work out what is most comfortable!

They are on the upper end of being kittens, about 7-8 months old now...
I feel really bad for Tiger, the person who adopted her said she had really settled in at their house. They were trying to cope with the allergies and hoping that they could keep her but it was just too severe. They already owned another cat and do not have much if any allergic reaction to her so were hoping Tiger would not cause one either. They said their other cat is missing her as well.