Yesterday we lightened a candle, Bianca and I. For the first time I spoke to her now I know she understands, and told her she had a brother and yesterday it was his birthday. She wanted to blow the candle but when I explained her it wasn't for her, she stopped . She said a couple of times 'yes, brother' and then went back to her toys.
In time she will know better, they will know better actually... that they had an older brother. It's not easy topic to bring up with a kid, but kids are amazing and surprise you in any possible way and I'm sure they will surprise me on this one as well.