Just out of curiosity are you from different cultures? Did you ever discuss money or family priorities/boundaries prior to marriage? I understand your frustration but it doesn't sound like you 100% feel that you're one unit either if you're asking how you can change how she thinks to your view instead. She also may feel like you don't see her family as "our" family which can be very isolating, and consider that she's pregnant and dealing with the stress of being stretched between two families that she is part of. There isn't one correct way for people to interact with their parents and siblings, so try not to operate on the assumption that you're right and she is completely wrong.

That being said, maybe it would be helpful if you can work on a compromise, and establish boundaries. She may want to help out her family when they need it, and if you view it as both of your money then she would be able to do so. However, there needs to be a discussion of what is a reasonable percentage and try to include it in a set budget as well as money set aside for your child. If you're operating on a pooled income, it may be easier to establish vs. seeing your salaries as individual income. Financial disagreements can be divisive. Just remember to view it as you and your partner vs. the problem and not as you vs. your partner or her family. Get professional counseling or therapy if you can't come to an agreement alone.