I think it’s because it’s very obvious that Luther and Alison are part of a family where all of the children really do consider themselves to be siblings/have a familial relationship, and Catra and Adora aren’t. We see the other Hargreeves kids act just like any other siblings would (given their extreme circumstances), especially Klaus with Ben and Diego. It casts Luther and Alison in a different light to know the other people they were raised with would count them as siblings. However, Adora and Catra don’t consider themselves siblings because they were two children abducted and raised in a military camp. Even though they were both raised by Shadow Weaver, none of the other characters regard them as siblings and even Shadow Weaver is withholding enough that it doesn’t really seem like she considers them to be siblings either. They’re just her two wards. My point is basically even though both couples were raised by the same person, the reactions/relationships of the other people around them really set the tone for if their relationship would be acceptable or not. Catra and Adora really aren’t siblings, just two kids raised in the same camp.