my dd is 4 and keeps talking about having siblings, she names them, she pretends other younger children are her brother/ sister.
she tells me to get pregnant and put a baby in my tummy
truth is I have miscarried twice since having her and 3x before she was born and at 40 now I really have to move on from this grief
so I tell her I understand her wishes, that she has lots of friends and family but that having siblings is unlikely, that sometimes it is not possible, but I do keep it short and light
be careful not to project your upset on her, she is likely to just want to be like her friends with siblings, it is only natural, but I expect she does not fully understand what it implies imo
but it is sometimes quite hard and you surely have my sympathy