I personally haven't heard of Sister Travel getting a remake, but currently there is no connection between the original Sister Travel outside of similar characters designs as much as some people say otherwise. Will they connect them? The DLC points in that direction, but until a remake happens any and all connections are simply head canon. Reusing character designs is not new with Inusuku, the idol girl Melody you meet in Sister Travel has the same exact design as Inusuku's first heroine of the first two Limit Oniichan!? series. She even wearing the same outfit.
As for Sister Explorer, it is as of right now a totally separate game. Hell, it actually predates the first Imouto Life game. Monochrome was started as a side project while Inusuku worked on Sister Explorer but then he really liked the idea and it turned into a full game. After that it was a series of push backs: Fantasy, Monochrome Multilanguage, The AI art game, Fantasy English version, Fantasy DLC, Fantasy DLC English version, Monochrome Encore..... so yeah, I wouldn't be surprised if Sister Explorer ends up becoming ST remake because that game is never getting finished.