Yeah... no... wait. At the first time I also thought "ah! touma die? That's sad, Mahiru is an orphan now..." but the again.. Mahiru is always an ORPHAN even when his father was still alive. Well, with Tooru he's just "kinda" orphan. And yes, Mahiru knows now that Touma is his father, but I think he doesn't have that "attachment" to Touma to have the willingness to stop him from sacrificing himself for Tsurugi. In fact... Touma is more of a father figure for Tsurugi than Mahiru.
And seeing from the timeline, Mahiru knowing Touma is his father was just a merely minutes ago. So.. building some kind of "A son who wants his father presents" feeling is impossible at the moment Touma decides to sacrifice himself. I think when Mahiru sees Touma made that decision, his very thought was "ah, finally he's doing the right thing", and being respectful person that he is, Mahiru didn't stop him. Why would Mahiru stop someone who's trying to redeem himself??!!
The moment when Touma blow smokes to Mahiru and he's saying "a kid should just be a kid. Just think about your summer homework" when Mahiru insisted to do the sacrifice thingy are ENOUGH for Touma to show that he is STILL Mahiru's father after all.
That's just my rant anyway. And besides, Mahiru has a LOT more people around him right now. That's enough for me.