Ok i can see why readers wouldnt like rayan and loina together but i think it works because of how they compliment each others dreams and aspirations. Plus rayan and tilly are technically 'siblings' (they arent cause the adoption got cancelled cause of the prophecy) because of the time they spent together as children. Its more of how close they got during that time and obviously loina couldnt form brotherly feelings for him or rayan for her cause they never met.
is it a little forced? Sure maybe but its more of best friend's younger sister trope iykwim. Plus considering that they arent related (even with adoption cause it got cancelled) tilly and rayan are just like best friends so close that they consider each other siblings, whicch is why so do tilly's parents consider him a son (yk like how you would to your child's best friend)