How the F*** do you do this with 2 kids?!!

We have a 8 month old and an almost 3 year old. We are trying to sleep train the 8 month old. She can get herself to sleep at night and during nap time. But during the night she just won’t settle herself. And it’s a coin flip on if she will take 15 minutes (quick5 minutes on the boob and back to her cot) or 90 minutes to go back to sleep. She gets no comfort from ANYTHING in the night other the boob. I cant just let her cry in the night because she will wake up her sister. So how the hell am I supposed to get her to go back to sleep.

I’m losing my mind. She is waking up after 3hrs 40min every night. Then it’s a coin flip of every 90’minutes after that or 4 hours later and I the barely sleep because I’m so anxious thinking about when she will wake up.

First nap is 1.5hours and second nap is anywhere from 30 to 1.5hours. Makes no difference on the night time. Her wake windows are 3/3/4

I’m exhausted, I’m cranky, I’m feeling the depression set in. I just want to sleep for a solid chunk of time (my husband sleeps downstairs and cannot help in the night due to work commitments)