Not sure how else to word that, this book popped back into my head just the other day and all the details I remember are odd. I would've read this around the mid-2000s, but I would guess it wasn't new when I read it - maybe from the 90s? I remember it being intended for a middle grade or young adult audience, despite being fairly dark and gruesome and explicit. I'm also 99% sure it had a sequel, but I either never read it or remember nothing from it.
I think, but am not totally sure, that the author was female. It had a very Pern/Valdemar/etc.-era vibe to me, but I don't think it was by Lackey or McCaffrey.
What I do recall is that the plot revolved around three children, a sister and two brothers. They're very close with each other but complete misfits at school - the girl in particular is heavy and tall, and gets a lot of "What's the weather like up there?" type jokes. (I think at least one using that exact phrasing.) The girl was the oldest, and I believe a teenager - I think the two boys were preteens or young teens.
One day, the three children somehow get sucked into the past and become embroiled in the lives of a Celtic or vaguely Celt-like (or perhaps vaguely Roman?) tribe that's in the middle of a war while trying to get back to their time. The book ends with them finding another portal, but cuts off before showing whether they actually made it home.
They are initially taken prisoner, and at one point are horrified to see decapitated heads attached to the bridle of one of the men's horses. It's said to be one of the heads of their enemies.
The specific detail I most recall is the following:
once in the past, the girl finds out she has some sort of natural talent for illusory magic; because women are treated very badly in this time, she uses it to project the image of herself as a male warrior. This drives conflict, with another warrior suspecting her and trying to expose her, and her little brother worried for her safety and also confused by his sister's growing passion for war and battle.
At one point, it seems they'll definitely be caught: after showing great prowess in battle the sister gets chosen for the honor of becoming a full-fledged warrior, which is some sort of ceremony that involves stripping naked and having one's genitals scarred or marked in some way that will definitely show the illusion magic to be false. The brother rushes around in an attempt to interrupt the ceremony and ultimately fails to prevent it - but is surprised when the ceremony goes through and the men don't suspect anything at all.
Later, the sister explains to the brother that this was her own doing - she's found a way to push the illusory magic beyond its previous limits, and has turned the warrior's body she projected into her actual, physical form.
This leaves the brother grateful for his sister's safety and impressed by her magic prowess, but also distressed; he begins to wonder who she considers her true family to be now, and whether she'll even want to go back to their own time at all.
(I suspect if the book were written today, this character would perhaps be portrayed as transgender - the character was always referred to as female as far as I remember, but I also recall a lot of "wanting to live as a man" as an underlying motivation, beyond even just a fear of how women would be treated. I don't know if the portrayal here was intended as an intentional metaphor, or whether such comparisons were on the author's radar at all.)
The two brothers also learned skills of their own in the past, I believe also relating to magic? I don't recall quite as well what they involved, though, except that I believe one had a wizened old woman who worked as the tribe's healer for his mentor during this. I also believe this world was intended as "a real time in the past, but with magic", rather than a fantasy world that happened to have similarities to Earth cultures.
The back of the book had a blurb for the sequel as well - apparently the plot of the second book would be that the portal failed to take them home, and instead brought them to yet another strange world, where they'd have to rise to prominence again. I don't know any more detail than that.