LaT - i think you have to look at 'now' as multiple wakings each night with both children to the 'future' when you (ideally) don't wake at all, or maybe just for feeds for D.
To get from here to there you may need to have several nights of bad sleep/hardly any sleep with multiple wakings from both DC, but hey you already have that!
Pick a plan and stick to it, you & DH. Make sure he knows exactly what the plan is, crucial to do the same.
Go in, ssh, settle her, tuck in, etc, no more talking & leave.
Give her progressively longer between 'sshhes'.
Use a kitchen timer if necessary.
Decide you are more stubborn than her, make it mental battle if it helps, 'i will win this, i am the adult, i have more determination' type thoughts.
A sticker chart for S could help as she is bright enough to get the idea.
Or; Say she wakes 10 times a night? you could try the 10 marbles (or something) plan, each time you come into her, she loses a marble. How ever many marbles left by the weekend = pennies/sweeties/etc.
When she wakes, you could even go as far as saying to her, i can come in but you'll lose a marble if i do? not everyone's style but if she's properly awake each time and wants the treat badly enough she may even tell you to go away!
Any of that helpful?
As for D, guessing he's young & could be waking anyway so get her sorted first. He may be being disturbed in which case he may settle once you 'crack' S.