I had a 3yo and 16mo when I had ds2 and agree that the tea - bedtime bit is the pits [understatement].
I coped with it for the first few weeks (up to about 9/10 wks, I think, when the evening bfing settled down) by reducing the older dcs routine to a minimum.
Baths only happened once or twice a week at most and I often ended up sitting on the loo seat feeding ds2 while they splashed. Towelling dressing gowns help too as you can just let them dry off while wrapped up.
I used to bf ds2 while doing the older ones storytime too - they cuddled up on each side as I was sat on the bed.
Teatime standards dropped so I did a lot of fresh pasta and sauces with frozen veg, if I was cooking I did double quantities so I could freeze half, pizza, fish fingers, sausage and mash, etc. Anything that took minimal preparation and could cook with little input from me so I could bf.
Or I did the main meal at lunchtime so I could just do toast/sandwiches etc at teatime.
If you can get a sling that you can bf in (Coorie is one) then it's possible to do the whole tea/bath/bed bit while bfing.
Just do what you absolutely have to to survive the next few weeks. A few weeks of less than perfect food won't do your dd1 any harm, neither will being a bit grubby!
This bit will be over very soon, in a few weeks dd2 will have settled into more of a pattern and everything becomes so much easier.