Will my female kitten accept the new male addition to the house?

We had adopted 2 wegies (Norwegian Forest Cats) 4 months ago... the male kitten died of FIP earlier last week... Since the previous owner had another male kitten we thought we should bring him home before the female gets used to being alone for too long... Seems that it was a misconception... as the new one is still considered the intruder... Deedee is getting used to Oscar being around the house but whenever he comes close she hisses at him... oh btw they're both 9 months old (same litter) He is very very very friendly and playful... sleeps the whole day and runs around the house at night... she is still playful and active but when he's close by she wont chase the laser light around :) but when he's playful and runs at her she hisses and shrieks (grrrrsss as well)... they dont fight and he is no longer scared of her... but she is still very very careful and the hissing is not stopping even though its been 8 days... what can i do to make this easier for her? someone please help me or give me a few tips :) thanks a lot...