Yu Ziyuan treated Wei Wuxian more like family than Jiang Fengmian did, and here's why.
(disclaimer that all of this explanation is quoted from Anon with their permission. I agree with them based on my own research and reading.)
If we go by what Chinese people tend to consider valuable in the modern age (and arguably before the modern age as well), then what looks to a western audience like a parent "favoring" one child is not actually a parent "favoring" one child.
What tends to matter to Chinese people when it comes to raising your children is education, discipline, and opportunities. A parent's duty is not just to love their child, but also to ensure that their child grows up in the right way, with the right values, right education, and ability to access the right opportunities. Unfortunately, for a lot of parents, this does mean being harsher towards your own child and comparing them to other children (e.g. 别人家的孩子), but the purpose of comparing your child to 别人家的孩子 is to prod your child towards working harder and developing in the right direction.
If a parent treats one 'Child A' leniently, but treats another 'Child B' strictly, it may seem from a western pov like the parent is favoring A over B. However, if the parent responds to transgressions from A with nonchalance, but seeks to correct the transgressions of B, then in reality the parent is favoring B over A. And the reason for this is that this indicates that the parent cares more about the future of B than they do that of A, because uncorrected transgressions lead to future problems and failure. You correct the transgressions of your own child, but you do not correct the transgressions of 别人家的孩子 because they are not your responsibility.
In this context of "favoring", we can say that it tends towards the conclusion that Jiang Fengmian did not favor Wei Wuxian for the above reasons.
Does this mean that Jiang Fengmian favored Jiang Cheng, and/or was a good father to Jiang Cheng? No.
However, based on the above, which parent is actually treating Wei Wuxian like their own child?
It can be argued that Yu Ziyuan ends up treating Wei Wuxian more like an "insider" / "member of the family" than Jiang Fengmian does. To Wei Wuxian she acts in the specific way that Chinese parents are to children they consider part of the in-group / their responsibility.