By the look of things, under normal circumstances he would've been sulking about it for awhile or made a big deal out of having to wait, look for the sister, and/or her odd instructions, which would've probably only made the both of them feel bad and potentially ruin their outing for the day. So instead of doing that, he decided to follow his girlfriend's example of keeping a cool head and positive outlook on the situation; which is good imo as the reason they're out in the first place is to go shopping for women clothes together (as an gift for his girlfriend, or so the Sister may think...), and wasting time sulking would be unproductive especially if his sister is there to help him in the first place.
Also, I honestly wouldn't be too worried about any incest routes, as from the looks of it from a previous chapter, his sister is also dating someone. So unless the sister is lying about it and had a crush on the mc since forever (which wouldn't add up with her proposing to go shopping in the first place), or there will be some crazily weird drama composing of love triangles and whatnots; I don't see any incest routes developing.
With that said, I'm definitely looking forward to more chapters.