Hi!
Your girls have now hit the teenage hormones. They happen in sows as well, but they usually not lead to outright fights so it is much less known. You have to judge whether it is just a temporary hormone spike, in which case you leave things be, or whether there is some long term problem in the offing that is going to rear its head again and again.
In sows over 18 months, you also have take the possibility of ovarian cysts into account when it comes to sudden aggressive behaviour because a sow is basically nonstop on season. However, this stroppy behaviour is more general and usually not targeted at just one lower-ranked sow in a group. Introdcuing a neutered boar into sow group will not cure a strictly interpersonal or hormonal problem.
However, bullying can happen and does happen in sow groups. Once it has developed to a persistent level over the course of a few weeks or months, the best solution in my own experience is to move the bullied piggy out and to find it a new friend, ideally a laid-back rescue dated piggy. A full-on case of bullying is usually connected with a weight loss from the constant stress.
When looking for a new companion for a sow of any age, you can look at either sows or boars of any age; it is the mutual liking and character compatibility that counts. Dating at a good rescue (if that is an option for you) allows you to let the piggies choose each other, so you come home only if there has been acceptance, and you have the assurance that any new piggy is healthy, properly sexed and guaranteed not pregnant.
Guinea Pig Rescue Centre Locator
By the way, I've just split one of my groups because of a bullying case in two 4 year old piggies (part of the same uncontrolled multiplying group that was rescues from horrible circumstances) that I adopted as a bonded pair. Although they have never been best of friends, three years on they have had a fall-out and the junior sow has been waging a sustained war on the other one.
I have introduced my bullied piggy and the bottom piggy of the same group to a mixed gender couple to form a new mini-group and avoid the potential outsider trap in a trio. This has thankfully gone well (the bullied piggy has not been able to challenge the lady in the pair for dominance). It also means that there is no longer a lower ranked piggy remaining for my bully to start in on now that her current victim has been removed. Instead of a group of 6 and a cross gender pair, I have now got two groups of 4.
However, I always give any moved piggies the opportunity to move back into their old group after a few days in case they feel happier with their old friends (a few companions have done so, but never a bullied piggy). Thankfully, neither moved girl did show any interested past saying a friendly hello to their old companions before trotting off after their new husboar when they touched noses through the grids with their old mates, so I know that I have made the right decision.
If you do not want to opt for an outright split, I would consider taking either the bullying or the bullied piggy out for a day or two for a next door trial separation and see whether your other bullied girl is suddenly perking up noticeably. If yes, then she has been feeling stressed. You can in that case consider whether you'd rather find a new mate for the bully or fr the bullied piggy. Your older sow as the leader is not involved in and hierarchy/bullying issues apart from the fact that it is stressful for the whole group. And you have to brace yourself that the bully won't happy if she's being removed, but her reaction doesn't count - you want to check whether the your little girl is feeling stressed out or notby the constant chasing.