21 month old hardly ever slept through, don't want to wake 3 yo with her crying, help!

[BeautyWithin6]

So I have a 3 year old DS who started sleeping through around 20 months old, didn't need to do any sleep training etc he just did it naturally. We have had rough patches here and there and currently since the dummy fairy came he doesn't like sleeping with his door shut anymore.

Across the hall is my 21 month old old DD, she's slept through occasionally but the longest being 5 days. She has a dummy, comfort toy, goes to bed awake and goes to sleep alone fine around 7pm every night. Naps 1.5 to 2 hours from noon a day. I stopped bottles 2 months ago but she has a sippy of milk before bed. I stopped offering milk during the night 5 weeks ago and only offer water and pats. Now she wakes every night often twice! If it weren't for my DS sleeping so close nearby I'd let her grizzle a bit but she's woken him before when I have waited 5.
I tried Ferber method but by day 6 she was absolutely sobbing and wouldn't even let me leave at bedtime it was like I'd lost her trust plus I had to keep shutting DS door and open it all night when she would cry it was awful. Everyone says just 3 hard nights and it wasn't that at all. Any advice for those that were in a similar situation?

DS started sleeping through at 10 months old, not 20. Typo.

Firstly, don't compare and don't assume anything you did or didn't do made a difference. They all learn to sleep eventually on their own timeline.

Do you think your youngest is waking hungry? It could be a development leap?It could also be that with the clock change her room isn't dark enough to suppress her waking and then she's awake with less melatonin at that point to just go back to sleep.

Neither of mine have ever and I mean ever "slept through" as defined by 8-6. We have always had at least two wake ups in that time. So what you're considering to be a problem is just the norm for some babies/toddlers.

I have black out blinds, noise machine on, she eats alot during the day and she slept through more often when I was giving her the milk if she woke overnight. Now that I'm patting her when she wakes she's woken every night since! Which is the opposite of what others experienced.
She wakes not at set times either sometimes around 11pm and 4am ish, other times just at 2 or 3am. I have heating on low in her room, sleep sack etc. It's like she's gotten used to the patting.