Are in in for a challenge?
You will need to prepare a safe room for the new baby. Put food, water, litter box, bed in this room (bathroom, spare bedroom that is not used). Make sure to spend time playing with the kitty as you don't just put the cat aside and ignore it. Start swapping scents. Then ou can bring out the new kitty and let your other baby smell it. You honestly don't know what the reaction will be. Some cats will hiss and growl. Others may warm up to each other pretty fast. The main thing is to not rush it. Your new baby will be stressed out from leaving mom and siblings, so it may want to hide. Getting new people, new smells and a new sister may be overwhelming, so patience is the key. After a few days, you can begin feeding them in the same general area. Bring out the toys and engage them both in play at the same time. You make the distraction the food and toys and they will usually ignore each other.
If your older kitty gets aggressive, break them apart and NEVER leave then alone unsupervised until they are best buddies. As for the litter boxes, I'm a proponent of putting them in different locations. For some people, that is not an option as they do take up space and you want them hidden away. If you have room for them together, it is probably fine, UNLESS you have a multi-level house or a large home. In that case, make sure you have a litter box on each level or in each end of the house.