Hello @Anonymous,
I changed your family and now it works fine.
You designed your fitting in the Centre(Front/Back) Reference Plane instead of in the Ref.Level plane.
For Pipe fittings that have to work within a Pipe Type you have to stick to the Revit rules.
If your family was just a fitting that you place somewhere in your project and you want to connect to it, then your family was almost just fine.
the only thing you had to do is set the Part Type of your family to Multi Port.

In the attachment, I placed my family.
Load that one in your project and you will be all fine.
Note:
I saw that you modeled a hollow elbow.
that is not necessary. You will never use that.
keep your families simple.
Model only the thinks that is really useful in your project.
Leave detail information that is not relevant out.
that saves time and is better for the performance of your project.
Hope this was useful for you.
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Kind regards,
Robert Klempau
Senior Consultant AEC
Cadac Group AEC BV