OP, they will only blame you if their father presents this as something that he felt pressured into agreeing to, and it would be difficult to imagine that they could respect a father who allowed himself to be pressured into having a child.
What exactly do you think that they will blame you for doing? Obviously, you did not hold a gun to their father's head and force him to provide his sperm.
It is much more likely that they just never expected their father to need or necessarily want another child. It is not often that you hear a parent of 3 saying that he or she really wants to have another child. It is probably the element of surprise and the unexpected nature of the situation that is informing their initial reaction.
It is also likely the the nature of the conception makes it a slightly different situation for them. Not many 13 year olds are very conversant with IVF as a process, and this may add an extra dimension of confusion for them.
A pregnancy is something that should bring joy to the expectant parents. Beyond that, everyone else's joy is only frosting on the cake .
Time and perhaps professional help may bring these children to a better place, but you cannot control how anyone ,particularly damaged children, will respond to a situation that they perceive as possibly having a detrimental impact on their lives.
You are in control of your own feelings and happiness, but you cannot dictate or control their feelings.
Step back and enjoy your pregnancy. Stop equating your ability to enjoy your pregnancy with their feelings about the pregnancy. It was never your intention to seek their approval to have s child. You have succeeded in doing that which you wanted to do, and they in turn will have to make their peace with your decision. Perhaps, that may mean moving back in with their mom, and like you, they too have to do that which they think best for them.
You all have many months to navigate all of this.