Did you breast feed your daughter?
I've got a new baby (11 weeks) and a 3.5 year old. I breast fed my eldest until he was 2.5 years old and although I stopped feeding him just before I fell pregnant he obviously has clear memories of himself being breast fed.
When I was pregnant with DS my breasts naturally started getting bigger and he started to show signs of interest in breast feeding again and asking if he could have some milk. I explained to him that he couldn't because my body was making milk for when the baby came and he then went through a phase of pointing at my left breast and saying "That's my milk" and then pointing at my right one and saying "And the baby can have that one." For the last 3 months of my pregnancy he kept asking me about the milk and if it was still his. I was very nervous about how he'd react to me breast feeding the baby when it was born.
Following the birth my DS came in to meet the baby and coincidentally I was breast feeding at the time and DS got really upset, crying and saying "Get the baby off my mommy, that's my milk, that's my milk" and it was quite distressing actually.
Thankfully when I got home on Day 4 my DS seemed to have done a bit of a U-turn (I think DH spoke to him a lot about it at home) and he then became fascinated with me breast feeding the baby. If I was feeding my DS would say "I want to watch him" and then just stand next to me and stare at the baby feeding. When the baby de-latched my DS would say "I want to stroke your nipple" but he never asked for a feed himself or make reference to the milk bring "his" anymore.
Every time the baby started crying my DS would holler at the top of his voice, "Mommy, the baby is crying, I think he wants some milk!!!"
I was very lucky that the baby was always a very quick feeder so I was never distracted from DS for long. Is your baby feeding frequently?
I find that now the baby has been here for 11 weeks and is having longer periods of being awake and needing my attention my DS's behaviour has deteriorated because he misses the one-to-one time we used to have.
Does your daughter go to a nursery at all?
Even though I was on maternity we kept our son in childcare because we wanted him to have some time where he could just be him, having fun instead of always being in an environment when he felt like he was second best to the baby. I found that at home I was constantly having to say things like "In a minute" "I can't play with you yet"
"let mommy just see to the baby" "leave the baby alone, he's trying to sleep" etc and it all just felt so negative and I think he felt a bit pushed out. When he goes to pre-school he doesn't have any negativity and can simply have fun all day and be focused upon in a way that he couldn't be at home.
Prior to having the baby people would tell me to make breast feeding a special time for DS too, I.e use the time to read a book together etc but I don't think that can be practically done. I don't know about you but I was constantly fiddling around with the baby, trying to get him to attach properly, changing positions, winding at frequent intervals, putting him back on when he came off etc and I don't think I could have been focused on reading a book to be honest and would probably have just irritated DS. But like I said, thankfully I didn't have to worry about this as DS was very accepting of me breast feeding.
I suppose it's difficult for your daughter as she isn't able to vocalise properly why she finds it upsetting, do you have any idea what her particular upset is about? Is it the act of doing it or resentment that your focus is on the baby?
Don't forget that it's still very early days for her to accept and adapt to a new baby being in the house. My baby is 11 weeks and my DS still shows signs of jealousy or annoyance regarding how much of my attention has to be spent on DS2 now and not just him.
I would recommend that you carry on doing as you are because it sounds like your explanations are good and the fact you watch YouTube videos of baby animals feeding is a really good idea - I think you just need to give her more time?
Has she been exposed to women breast feeding at all or is it a new concept to her?
Maybe you could watch videos of women breast feeding their babies so she can see that it's something normal for people to do af well, not just something animals do as that may be confusing or upsetting her as she won't understand that as people we are also technically mammals who feed their young too.
Sorry for the length of this post 