I think you are overestimating what multisampling is meant to achieve. As the term anti-aliasing kind of gives away, it is a technique that is meant to counter aliasing. If you don't suffer from this "problem" then there is no need to employ any form of AA, be it MSAA or SSAA. As long as you are rendering rectilinear polygons that are not rotated by non-right angles and assuming that their texture contents don't exhibit aliasing, then this problem simply cannot occur. I assume that your minimap, as is the case for most minimaps, will also be a rectangular region somewhere on the screen, meaning it will not benefit from AA at all.
Following from what I said, AA is only required when rendered polygons don't line up with the target pixel grid, thus necessitating sampling multiple pixel locations in order to determine the final framebuffer value. In order to get into a scenario where AA in RenderTargets might be necessary, one would have to render multiple rotated polygons into a RenderTexture, perform some post-processing and then finally render the output to the window framebuffer. It is important that the drawn primitives themselves contain unaligned edges. If you were just drawing sprites that rely on an alpha-blended texture, AA wouldn't help you much either. In this case, it would be smarter to pre-process the image data offline to smooth the edges. SFML also lets you get texture "smoothing" by choosing whether to employ linear magnification and minification filters to the texture data, but this is a completely different problem from aliasing.
RenderTextures should really only be used when any form of intermediate compositing or post-processing is necessary. I've seen enough cases where people have misused them where a simple viewport/clipping would have sufficed. The problem is that SFML doesn't adequately document the cost of these graphics resources, so beginners are inclined to believe everything is just as "cheap" as everything else. RenderTextures are quite a heavy resource because it among other things requires creating a new context. Unless you absolutely need to use them and there is no other alternative, I would always opt to use the cheaper solution.
Take a look at the sf::View tutorial, it even explicitly explains (with pictures) how one would use them to create a minimap, all without a single RenderTexture.
