EmmaPF wrote: …Can you set a birthdate for photo recognition?
No, you can't. You may need to re-teach Photos the difference between them, though.
The first thing to do is to make sure they all have the right name tag. On ones that are wrong you can touch the little circle and choose "This is not …" That will help remove some of the facial characteristics that caused the confusion. Then you can add the correct name to add those characteristics to the right person. This will probably work, but Photos may be so confused that it will take more work.
I suggest that you add the names to the Caption fields like this:

I have her name, and I added other stuff, as well, because I can't keep up with all the relatives.
A search will find the person by name, so if the People Albums stay confused, you can get them back.
If fixing the names doesn't work, then the best thing to do is to start all over, remove all the names names of these two and replace them with new names that have never been used, like Katy#2, or something. After Photos has had time to figure all this out, you can change the names back to ones you like better.
Let us know how this goes…