You have my sympathy - it is really hard with a bf infant and an older child. DD is 6mo today and DS is 3 next week and it is better, I promise:)
We didn't try forcing DD into a routine - it was actually easier I found to just go with the flow and accept it would be random rather than trying to worry about feed times. We did do 2 things from about 6 weeks though to help her
We followed the Gina Ford advice (not her routine, oh no no no no, no way) that new babies can't go longer than 2 hours without a nap. That has really worked for both of mine - just settled baby in basket or wherever about 2 hours after they have woken up. It is a miracle - baby just settles easily. Meant she had 3 decent naps in the day and wasn't overtired which helped her settle better in the evening.
From 6 weeks, we also gave her a bath with DS and got her ready for bed then she came back down with us - some nights she was up feeding until we went to bed, other nights she would drp off around 9ish. By 12 weeks she would come back down with us but fall asleep at 9 and sleep until 7 no problem, so then we started putting her down after bath at 7.30-8 and she would go at least 10 hours without a feed.
It all went to pot at 4mo when she hit that 16 week growth spurt/developmental thing, but at 6mo we're only one one night waking now - mainly because she is oinly just on solids so quite hungry - and a good solid bedtime routine with no fuss.
Be kind to yourself and don't worry about your older DC. My DS has survived what seemed to me huge neglect (given he was the previous only focus of my attention) and gets much more attenyion now DD is older.