Yes, it'd definitely be realistic, if done a certain way. For example, as you probably know, Christianity does have many branches beneath it, but there are three different religions that share very many characteristics: Islam, Christianity, Judaism. Now, I can't say much much about it because my knowledge is limited to what I learned in a college class, so you might have to fact check me, but they share, for example, the concept of Hell (Hell, Gehennom, Jahannam, respectively.) They also all have one god (God, Jaweh, and Allah), and an End of Days, among other things. They are all Abrahamic religions and it's said they descended from ancient Judaism, so this is why they share so many similarities. Regardless, they're clearly very distinct religions (to the point we've fought wars and continue to have tensions over them) and aren't categorized as branches of each other.
Now, I don't think it'd be plausible for vastly different religions to have the exact same God, because religions usually separate due to differences of belief and interpretation. Additionally, if you have sects that have been secluded from each other, they're inevitably going to deviate drastically. Kind of like how certain species can evolve to be so different from each other if isolated, despite coming from the same base. An example:
Say you have one sect isolated in the mountains. Their problems will be vastly different from a sect isolated in the savannah. One will be unable to farm their land because of rock, maybe snow, maybe a lack of fertile land. So they would pray to their God for this. But a god who doesn't exist can't help them, so nothing happens, and they begin believing their God is a harsh one, one intent on challenging His people. The savannah people, however, have issues of drought and they ask for rain. They pray and offer sacrifices, eventually they get rain. So they think their God is one pleased by blood. Or something like that
Also, secluded peoples will form differences in language, and this can affect religion as well (hence, God, Jaweh, Allah.) So that could be used to differentiate between them. But also, it's your story, and it's in a fantasy setting, so I think you could really get away with a lot.