Alienated 18yo - what to say to half sibling

[LittleLionMansMummy]

Dsd1 went nc 4 years ago when she was 14. I don't want to retread old ground as that's not what this is about.

Ds was 1yo at the time and they'd been very close until her departure (his birth was not the catalyst and for a short time she continued to see him if nit us). Obviously he doesn't 'know' her now but he recognises her when he sees her in the street (he attends a cm who lives close to DH's ex). Dsd1 spoke to ds's cm the other week while he was with her (she didn't speak to ds or acknowledge him) and when she left ds apparently said "that's xxx, she's my other sister" (dsd2 is still very much part of our lives).

We still have a photo of dsd1 on our wall and although we don't speak about her much, I do want ds to know she exists as when he's old enough he might want to try to contact her. But I don't want to make a big fuss and a big deal of the fact he has another sister in case she never comes back - I don't want to raise any expectations in him. We also have the added complication that dsd2 is still part of our lives so ds's relationship with her is great and hopefully always will be, while he has no relationship at all with dsd1. Additionally we can't pretend dsd1 doesn't exist because it wouldn't be fair on dsd2.

It's a matter of time before ds starts to ask questions and i just don't have a clue how to respond to "why don't we see her? What happened? Will I see her again?" Etc.

Has anyone else experienced this or have any advice on how we should handle this?