I’m gonna start off by saying yes I know I didn’t introduce my cats right but now I need help fixing it.
I have had my cat Festus since April 2017 he’s about 10 months old right now, I love him with all my heart he’s my baby. However I always knew I wanted two cats, so when I had the opportunity to take a kitten home with me one night I took it without any thought and amy hestitation. So now I have Midge a 2 month old kitten. I brought Midge home the same night I met her and immediately brought her upstairs into my room, Festus’ territory. I held onto Midge and let Festus approach her and at first he got close and sniffed her with no signs of aggression and then 30 seconds later it was hissing and growling. Festus hasn’t actually attacked her yet but now he avoids being in my room at any cost opting to hang out downstairs by himself. It’s really disheartening because he’s my baby and I would never want to upset him.
As far as separating the cats it would be hard considering I live in a house with other people who wouldn’t necessarily want a litter box and a kitten lingering in their bathroom for up to weeks while my cats get acclimated.
Midge wants nothing more than to play with her older brother yet he wants nothing to do with her. What can I do to help Festus warm up to the idea of Midge and want to spend more time in our room again??
Don’t beat yourself up - you’ve have given two kittens a forever home. Yes, many would say other methods may have helped the ‘sibling’ process but this will work out with some extra time.
They have separate litter boxes ? Beds ?
Food bowls ?
I would encourage a joint dry food bowl and a joint water bowl.
Brush them together - allowing them to wander to your brush and away … takes practice but soon they will both be begging for your brushing attention and bumping into each other in the process.
I would feed some smelly wet food on separate plates - plates getting closer gradually over the days … then when the warming up is solidifying …one evening when you are in the room with them … present a little canned tuna / packed water (2 table spoons) spread out on a large plate ( dinner plate) … spread the tuna around the edge of the plate in a circle … they should scarf it up
then encourage some brushing time on the bed together.
No need to worry - they are young and different sexes - time will bring them together for wrestling !
Good on you for taking them as yours ! 
Google Jackson Galaxy
Older cats usually hate kittens at first. I would not separate them, let them work it out. It is a male and female so they will eventually get along. Ignore the growls and hisses. it is normal. As long as he is not hurting the kitten he will get over it. you might find they will become friends before you know it. Let him sulk. Make sure you have at least 3 litter boxes. Kittens can be a royal pain to older cats, they have no respect and want to play, but your guy is young. I have been through this so many times. They always get over it. Try to get some interactive toys that you can play with them together. like Da Bird. 
Agree with Sannois. Get them both engaged in playing with something. Da Bird is a good idea. I fed my two tuna and they got over themselves really quick. Don’t leave them unsupervised for the young ones sake.
While I’ve found the 8 months to be the oldest age where a cat will accept another cat or kitten, 10 months is not that much older, so just give it some time. One day you’ll suddenly find them sleeping or playing together.
