Technically you can't even ship those two together. Considering Senketsu is made out of Ryuko's genetic material so that kinda makes him her brother/son. I think that's how it works, but I'm not sure.

I don't think it's fair to apply human moral values to sentient clothing. I think the fact that Senketsu is a shirt is a bit more of an obstacle than the fact that he is spliced with parts of Ryuuko. He is his own personality and didn't grow up with anything, and he is built from life fibers instead of human DNA, so I wouldn't call him a brother or anything like that.