Hi No2, congratulations on the birth of your DS.
I have an 18 month age gap between my two boys, DS1 was fairly chilled out from the beginning, more fascinated by him and too young to now remember a time without him. DS2 is 4 months old now and DS1 will run and get a dummy when he cries (though DS2 is a thumb sucker now), comes and tells me if he's crying and bundles him on the floor when DS2 is laying down and asks for cuddles with him and loves being a helper - when I've finished changing a nappy DS1 runs off with it and throws it in the bin!
However, he will hit DS2 if he is cross with me in any way, telling him no for something, has never had a problem with me bfing, but will do naughty things whilst I'm feeding cos he knows I can't get up and stop him! DS1 does get jealous, its all for my attention, today he hit out because I was holding DS2 and he wanted to read a story!
Your DD will get used to her brother quickly, one week isn't long. Do you have a box of special toys you can get out when it's feed time? I found I could devote time to DS1 once DS2s sleep was better and feeds didn't take so long, but our experiences sound very different, it depends on the nature and personality of the older child!
Just take things as they come, CBeebies is your friend and tbh your DD will go through developmental leaps of her own, at 22 months DS1s vocab has just exploded, so with time she will be able to express herself better and tell you what she wants.
What I'm saying in a very waffly way is it totally depends on the child but your DD soon won't remember a time without him, maybe involve her in bathtime (we wash baby's hair together).
You are not letting them down, it is one week in to a big adjustment for you all, my HV just said to me they both have to learn to cope without your instant attention, and babies and toddlers have to be the most self-centred beings around!
Hope things get easier for you, they will, it may take a few weeks but then you'll see the two of them together and it will make you smile