Remind me but how does pregnancy ending causes the imouto to die? I know that it definitely carries a huge risk considering her special constitution, but it never explicitly said she died or would die as far as I remember. It could be me simply remembering wrong though, or failed to read between the lines.
Anyway, I've been aware of Sister Travel but been hesitant to pick it up considering I'm not even done with several games. Would you recommend it?
First, yes give Sister Travel a go. It's going to annoy you having to launch it with the steam emulator, the low res is a bit ugly at times, and it was made back when Inusuku didn't know how to program mouse input, but there are a bunch of cute girls, the MC is a dumb horny funny cinnamon roll and now with the context of Fantasy, this is how you have sex with Kana after only having dream sex with her in fantasy.
As for pregnancy, it's purely a matter of the numbers. From 'full hp' she should die from 1 night of 20 orgasms. Without being physically cared for she decays to death. Someone that weak isn't going to survive pregnancy without at least a doctor, and in a time of the world where horses were the fastest mode of travel? Nah she's dead.
And the game just ends at pregnancy ending. Why wouldn't he finish off the grand adventure afterwards and give her enough life force to survive another 10 years...unless he couldn't.