Celebrate the Baby, but not the sin.
Any gifts you purchase should be directly for the baby. Anything you say could be direct support of the child.
If you are close enough to your first cousin (or if your cousin knows your stance on in vetro) you could mention something like “while I do not agree with the practice of in vitro, I’m very happy that a new life is entering our family.”
When people (who disagree with Church teaching on in vitro) ask me how I cannot be in favor of a parent having “their own child,” I always say because I believe that there are many children who need families around the word via adoption and that their are many millions more who are aborted who should have been available for adoption. When good parents choose in vitro, another child potentially misses out potential great parents. So it would be hypocritical of me to be against in vitro when it’s a stranger and pro in vitro when it’s a family member. I also believe that abortion creates the “need” for in vitro. If abortion was illegal, there would be millions more children who would need to be adopted… in vitro would most likely lead to a large number of those children not being adopted. The abortion and in vitro industries are very closely linked.
If you are not close enough to say something, simply express your joy for the new life, not their actions.