I think Swansong dying after killing Leopardstar the second time could be a good spot. A swan song, if you will. Like you said; his sister and brother had the shoulder what Leopardstar did to them alone (and Stonefur died because of it). I just imagine Mistystar seeing him in his last moments with all the love he can muster for her sister before finally going night night forever
It could also tie nicely into the very somber, reverant way she responds to Lizardtail coming to fetch help.
She's just been sitting beside Swansong, trying to apply pressure to his wounds, but there's just too many and there's not enough herbs to go around. They both admit that nothing can be done and just choose to spend their last moments together saying goodbye.
When his final breath hitches in his chest and his flank goes still forever, the noble leader finally allows herself to let out a soft sob. It's been a long time since she's just let herself cry, even a little. For almost as long as she can remember, her life has been like someone hooked a claw into her belly and she's slowly being flayed alive.
Oakheart, Silverstream, her uncle Crookedstar, Graypool, Stonefur, Pikepaw, Primrosepaw. The killing of Deerfoot for the crime of saving her life, the betrayal of her mate Blackclaw, the furious battle with Leopardstar for the fate of her Clan.
And now she has outlived both of her brothers. Her fiercest, most loyal ally, who has been with her through everything, is gone.
She couldn't be cruel to him as he died, wouldn't let his last living memory of her be anger, but now she lets herself sob and clutch his white fur in her claws. How dare he die for her? It was always enough for him to just be alive!
By the time she regains her composure, Jayfeather still isn't done with his... whatever he's doing. But they do see the dark water of the lake rippling, and Lizardtail bursts onto the shore and bounds across the moor, wheezing about Tigerstar's change in plans, begging for help.
When he collapses right there, exhausted but alive, how could she not admire this? ...some may want to call him running away an act of cowardice, or dismiss the Dark Forest trainee as just trying to save his own pelt.
But she's just lost her own brother. Death doesn't seem so glorious right now.
No, she remarks quietly, on this night, he has made a hallowed flight across the lake. He's come for help, and his information will save many lives. She doesn't have the idea of making this an Honor Title just yet.
It's just very, very brave, in her weary eyes. Enough loss. Enough proving our mettle in battle. Let's end this, so that we can live.