yeah it is the case but. they are making it clear that the mc and the princess will always be together, and in the last page the mc's flashback of the other princess saying to her "what if she was devoid of any magic when she needed it the most"
unless it's just a bad translation, the other princess was talking at the mc, and she said the line above not "what if you were devoid of any magic when you needed it the most", also the word "devoid" makes it seem even more like she was talking about the princess running out of magic.
I do see what you mean. First tho, they can't literally always be together, they aren't bathing or sleeping together (yet). Second, Eldora is saying Tamura doesn't have her own permanent source of magic. A more literal translation of that line is something like "because she has no magic power, she couldn't do anything at a critical moment, could she?". Maybe "what if [Tamura] was devoid of any magic [source] when she needed it the most?" is clearer but I think my original line is still fair.
Also in JP there is no subject-verb agreement so it is unclear whether she is speaking
to Tamura or
about Tamura. But given her disrespect of Tamura (including previously choosing not to address her directly) and the way the lines are written, I went with she's speaking to her sister, about Tamura. Either way it's consistent throughout her dialogue, in the TL she is referring to Tamura, in 3rd person, the whole time.