Aaaw, she sounds really lovely, do hope she settles in well.
I would definitely keep them separate for some time though, there is no reason they shouldn't get on, but equally there is no reason that they will either. Like us, guineas need time to get to know each other and may not instantly bond. Your older one may be very stressed by the new interloper, and likewise the younger one may be freaked out as well. Introduce them very slowly. Keep them separate but within sniffing and squeaking distance, and introduce them in neutral space which is not fenced off in anyway, so they can both escape and hide and and when they needed to. I have done this many times, and the secret is not to rush or get impatient, they probably won't share the same cage for some time, if at all. Ideally, I would let them have supervised free range time together in the garden if that is at all possible, to get used to each other for a while. You will have to watch them carefully together for at least a few weeks - give them separate food bowls etc.
One bonding tip is to put them in a neutral area, and give them a MASSIVE pile of grass/dandelions etc in the middle so they will be distracted by that. IF and and when they become agitated or aggressive, you need to separate them and then try again some time later.
It can be time consuming and frustrating, or it can be easy, alot depends on the nature of the sows, dominance type issues, territory issues, and animal instincts etc. Good luck. Look forward to hearing how it all goes 