SMA Tee and impedance matching
When SMA Tees or BNC Tees are designed, is everything 50 ohm impedance matched?


No, a simple tee is not impedance matched. 

Looking in from the source side, you see two 50-ohm outputs in parallel, giving roughly 25 ohms load. 

Similarly, if any of the downstream loads produce a reflection, when that reflection returns to the tee it will see a roughly 25 ohm load, resulting in a returned reflection back upstream.

If you do not want multiple reflections causing issues in your system, you should used a matched splitter of some kind. There are multiple types, with the best one for your application depending on what frequency band you're working with, how low you need the reflections to be, etc.

Possibly the simplest kind to understand is the resistive power splitter. This has the advantage of operating over very wide bands, but the disadvantage of absorbing half of the incident power in the resistors, resulting in 6 dB loss from input to output, rather than the 3 dB you'd get if power were perfectly split and delivered to the outputs. 

More complex types of splitters are generally transformers of some kind, using either magnetic devices or transmission line structures, depending on the operating frequency. (Note a transformer based splitter will produce a phase difference between its two outputs --- possibly this could eliminate the need for the separate phase shifter device in your system, depending on exactly what you're doing and how accurate you need the phase shift to be)