Your tweets on both tenma siblings being lonely are so true because we often see takes that tend to act like their parents were only with saki when they weren't. On tsukasa's side I think the decision to make his feelings happen during a time where its normal for children to feel jealous towards their siblings for getting more attention than them is really important. And because of that i think its honestly very likely that he would/could have felt that way even if saki was healthy and their family was together. As for saki because her illness is something that no one has any control over her feelings of loneliness are largely tied to the fact that at the end of the day they have to go home. Her family and friends can't be with her 24/7 so there are inevitably times where they leave her alone.

As a result we end up in this interesting situation where tsukasa's loneliness is the result of the actions of others (his parents unintentional neglect) while saki's is the result of an uncontrollable situation (her illness). And yet tsukasa never once blames saki for anything, he expresses that he felt jealousy a single time but even from a young age he knew that it wasn't her fault. While saki views herself as the reason why her parents cant be with her all the time and why they can't be with tsukasa all the time. Tsukasa views saki as a victim of her illness while saki views it and by extension herself as the cause of problems for their family.