Note for idiots: In the real world, when judging what is and isn't appropriate, whichever is the younger out of physical and mental age is used (i.e. usually their physical age, unless they've got brain-damage).
For isekai characters, that generally means that what is appropriate for their age is determined exclusively by their apparent physical age. It doesn't matter if they reincarnated in their 30s or whatever. That is NOT A FACTOR.
In this case, it would still be skeevy because, in so far as their incarnations there go, he is 15 and she is 11. Her reincarnation and his age-down are non-factors. Since he is deemed to be an adult at 15, that means it would be fine for them to have a relationship in 4 years. That said, he is obviously more interested in the other girl at present.