Thank you for explaining this situation. How unfortunate that this share joyous event is turning into a competition.
The experience of being left out or ignored as part of the situation is what needs to be addressed. First, I would have a talk with your fiancé about what is happening and why. Does the family have a bias against the pregnancy because you were not married first? Is your fiancé on the outs with his family? Do they have a resentment against you for some reason? The key to understanding why this is happening is in understanding the reasons behind it. Of course, they may be superficial reasons or unconscious ones, but having a talk with your fiancé about this may be helpful in uncovering being slighted. This can also help decide the best mode of action for remediation.
The fact that they are treating you both so differently may not be something they are consciously aware of. As a way of fostering a connection, I would include yourself into the mix and have your fiancé facilitate this. If they are touching her belly and not yours inviting them to feel the baby might be something you and your fiancé can offer. Join in the attention to your future sister-in-law and invite her to share time and information with you. Make yourself and your willingness to engage known to them.
It way also help for you to take on a role where you are asking questions of your sister-in-law or other members of the family. You having this need from them may be part of what shifts the dynamic. Discussing this with your future husband can give you both a sense of connection during this, as well as support.