The division was created when OP had two children with two men whose families would not offer their respective relatives the same. Trying to pretend that they are the same, and trying to force others into supporting that narrative does no one any favors.
I don’t hold the nuclear family to be ‘most important’, no. Family dynamics and nuanced and complex, particularly with blended families, and sweeping absolutist statements that don’t reflect that are rarely helpful to the individuals that make up those families.
Op’s youngest has been raised to be close to her paternal family, with her father at least actively encouraging it. Like it or not, her relationships with them are important to her, and she should not be expected to choose between her extended family and the one she lives with. It is not her fault that her sister does not have the same, and it is not her responsibility to give up her own for her sister. Harming her under the guise of ‘protecting her sister’, no matter how much you try and spin it into a positive for her, will not make anything for better, for her sister or anyone involved. Again, she is a person independent of her sister, with her own thoughts and feelings, wants and needs. Her sister is not the main character her life must revolve around.
She has been raised with this dynamic being absolutely normal to her. She knows, as her sister does, that her paternal family is not her sister’s and has no problem with it. She’s not going to suddenly change her mind because her mother wants her to now.
There is no reason for this wedding to wreck the family unit, any more than the last one did, or any more than the youngest being in and out of her grandparents house weekly, when her sister has little to no relationship with them, does. As if preventing her from going (if OP even could prevent a determined 14 year old from attending, especially when she has the support of her father) wouldn’t do the same amount of, if not more, damage that you envision her going would.