Hi, K.. Well, try not to rush getting the older child into a different bed than what he is used to. Then again, you don't want to make him jealous by him seeing that you are taking away his crib and immediately putting his baby sister in it. So you need to establish a balance over time before making this transition permanent.
Does your son ever fall asleep on the couch or in something like a big recliner chair? If he can nap in something other than his crib, or even sleep through the night already in something other than his crib, then he might be ready to go to a youth bed. However, at 14 months, I would DEFINITELY use a bed rail to keep him from falling out of bed. These metal rails are shaped so that they are anchored under the mattress, and they take up about half the open side of the bed so that if the little one rolls over, the bed rail will catch the child and not allow him to roll off the bed. Be careful what you purchase because they are usually made for kids over 2 years old, and you don't want to get one that is so big that the child's head can get caught between the rails.
See if he would be happy taking a nap alone on your bed during the day, and see if he feels secure. It can be scary for a small child moving from the closed-in security of crib walls to a wide-open space like a bed. He is old enough to understand, too, that he can fall off the side of a bed, and this can give him anxiety. If he can nap happily, try setting up the bed somewhere and seeing if he's happy sleeping on it at night. Give him lots of praise for sleeping outside the crib. Let him know what a big, strong boy he is!
If he can transition to a bed well, I have another suggestion: dismantle the crib and let him get used to the bed at least a few weeks before you put the baby in it. You absolutely do not want to have your son feel like he is displaced in favor of the baby. Make sure you pay him lots of attention so that he is very secure in the fact that you love him as much as ever, even though he is making a transition.
I hope all goes well.
Peace,
Syl