Monsters-as-PCs causes this
Unfortunately, the rules for monsters-as-PCs don’t work. This is basically what you get when you use them.
Consider: a nymph is CR 7. She casts spells as a 7th-level druid. She doesn’t have the druid’s other class features, but those are mostly pretty mediocre—wild shape is good enough, nature bond is OK, but everything else is mostly just fluff (“ribbons” in game-design parlance). The big draw of the druid class is the spellcasting.
So by being a nymph rather than a druid, you effectively trade all your mostly-mediocre druid class features, keeping the by-far-best one, for colossal ability score bonuses, a paladin’s best class feature plus a far-superior version of one of the best of an oracle’s revelation options in unearthly grace, incredibly cheap crowd control in blinding beauty, and stunning glance for when she doesn’t feel like wasting a spell slot or getting her hands dirty. Inspiration is top-tier support for one ally, that requires zero effort on the nymph’s part. And she gets dimension door once per day because why not?
In short, if this was presented as an archetype, would you allow it? No, you probably would not. It would be preposterously overpowered. In some significant ways, this is a druid plus a paladin plus an oracle, plus absurd ability score bonuses.
Monsters were never intended to be played. Pathfinder even went so far as to eliminate shapeshifting options for doing it, in what was easily the best improvement on 3.5e to be found in the system. That means that CRs were not set with PC use in mind. It was supposed to be impossible.
So now what?
Unfortunately, you have already started a campaign with this as the premise. Your options for this are:
Just bite the bullet, accept it as a lesson learned, and just try to make the best of things. In combat, maybe just try to walk right past the nymph (who, despite everything, has actually squandered a lot of the opportunity here, and so has little ability to actually stop people from ignoring her). In social situations... maybe have people recognize her supernatural allure and be unwilling to work with her, forcing the other characters to handle social interaction? That kind of sucks, but it makes a certain amount of sense and one person being left out is probably better than everyone-but-one person being left out.
Accept it as a lesson learned, and end the campaign one way or another, and start over with a new one, not allowing monsters-as-PCs. Or even contrive some reason why the current PCs (at least the monstrous ones) have to be retired, in-character, and have the affected players write up new, non-monstrous PCs.
Attempt to homebrew some non-monstrous nymph.
Homebrew non-monstrous nymph
Here are my thoughts on this nymph:
First, for the race, we want a typical, playable race. Net +2 to ability scores: +2 Cha, +2 Dex, −2 Str perhaps. No native spellcasting, no dimension door spell-like ability, no bonus feats. No unearthly grace, it’s just far too good. If any DR is retained, it’s DR 2/cold iron or DR 3/cold iron, at most. Wild empathy can stay, as can the +6 bonus: it’s huge but also, who cares? Then blinding beauty, inspiration, and stunning glance can become daily-limited abilities, perhaps something like
Inspiration (Su): [...] As long as the nymph retains her favor for this creature and as long as the creature carries the nymph's token, the creature can gain a +4 insight bonus on a Will saving throw, a Craft check, or a Perform check. A bard who has a nymph for a muse in this way can instead use his bardic performance for an additional round in a day. However it is used, the nymph’s favor can only be used a number of times per day equal to her Charisma modifier.
Honestly, even that is really good. Once per day would not be unreasonable.
Stunning Beauty (Su): Once per day as a standard action, all humanoids within 30 feet of the nymph must make a Will save (DC 10 + half the nymph’s hit dice + the nymph’s Charisma modifier) or be blinded for 1d3 rounds. Creatures can avert their eyes to avoid being blinded, as if this were a gaze attack.
In addition, the nymph may single out one creature within 30 feet while using this ability. That creature must save (again, if it was already subject to the blinding effect), or be stunned for 1 round. This effect may apply to non-humanoids.
This is a powerful effect, but at 1/day it’s limited enough that maybe it’s OK. I wouldn’t feel too bad ditching the stunning effect, though, to be honest. That blinding is already really powerful.
Then we can have a druid archetype for her. Note that several oracle revelations offer Charisma-to-AC (but either replacing Dex and limited by armor’s max Dex, or only while unarmored), and the paladin’s divine grace offers Charisma-to-all-saves. Enlightened paladin gets both divine grace and confident defense, which is Charisma to AC while unarmored. But since this character is heavily-armored, allowing a druid to trade nature bond, trackless step, wild shape, and woodland stride for the nature’s whispers revelation from the nature mystery, and
divine fey? grace from paladin, would be... well, it would still be ridiculously good. I probably wouldn’t allow it. But it would be closer to fair.
Then you could have a nymph 7th-level druid/5th-level fighter that is far closer to a balanced player character.