I don't mind the sex scene itself (there's been plenty of those in this manga already anyway) but i do feel like Helena specifically sleeping with Eliah does feel kinda gross since she's up to this point felt like more of an older sister figure to him. Although of course knowing this author that was probably the point. It reminds me somewhat of Misato and Shinji in the latter episodes of NGE and in EOE, where, because of how their lives have been, the only way they can think of comforting the boys after a traumatic experience is through sex. Except in this case Eliah is much more reciprocal than Shinji.