Now I know this might not be "official" when it comes to where Sinnoh is in relation to all of the other regions. But I can't help guessing that Sinnoh might be located north of the Kanto/Johto landmass.
Here's my evidence:
1)Sinnoh has a cooler climate than all of the other regions and should in theory be located in the north.
2)The biggest piece of evidence to me would be the location of the Sinjoh Ruins that are located in an area north of Johto. Now if people from Sinnoh and Johto settled that area that must mean that Sinnoh is not too far away from Johto.
-Just say what you think. (Oh and if there is already a thread about this somewhere or if I put it in the wrong Forum Topic, sorry, I am still new at this!) :D