MC's sister wakes him up.
mc「なに……してるんだ？」
sister「先に目が覚めたから、お兄ちゃんの寝顔を見つめていたの……♪」
全国１０００万の妹好きを下僕に出来るだろう、笑顔と台詞が返ってくる。
..., i reply with a smile.
i an assume this is saying in a roundabout way that he is a degenerate for his sister, but hard to interpret this statement
thanks
First, you should not ignore てくる. This sentence lacks a personal pronoun, but it clearly indicates 笑顔と台詞 is returning toward the MC, i.e., she replied with a smile.
Second, 全国1000万の妹好きを下僕に出来るだろう is a relative clause that modifies 笑顔と台詞, which serves as the subject of 下僕に出来る. Don't be misguided by the comma.
Third, AをBに出来る is a potential version of AをBにする ("to make A B", "to turn A into B").
Therefore, a literal translation is:
A smile and words, which could probably turn 10 million imouto fans in Japan into her servants, returned to me (from her).
Or more naturally:
She replied with a smile. Her smile and words could captivate 10 million imouto fans all over Japan!
全国1000万の妹好き ("10 million little sister fans in Japan") may need some explanation. It's basically a reference to that "imouto moe" or "imouto-ish character" fad, which was most popular in the 00's and has little to do with incest. The relative clause describes her response was that of a stereotyped 妹キャラ admired by many otaku. It says her smile was objectively fascinating, but it doesn't necessarily mean MC himself was fascinated (at least at this point).
