i need help with my cats

so basically i just got my fur baby (kitty) last monday and yesterday my mom dropped her cat off because she went on a holiday for 10 days and we don't live in the same country. when we released moms cat, she ran under the bed and stayed there for a few hours, which is totally normal since it was a new place for her. the problem began when she came out and started hissing and growling at me, my boyfriend and the kitten. for safety reasons i decided to go sleep in the guest room with the kitten, since the moms cat was so aggresive. the aggresion stopped when i was alone with her, but she could hear the kitty meowing and started hissing at the door. i really dont know what to do here because my mom will be gone for 10 days and i dont know if its okay if i just keep the kitty in the guest room (of course i would come and play with him and sleep with him, since he is very attached to me) because i dont want them to fight and i want the moms cat to relax and be all cute like she usually is. do you guys maybe have any suggestions?