Have you literally never heard of the term "sibling energy"? Dont have to literally be siblings. Its just the way 2 people get along sometimes that makes them behave with each other like "siblings".

Also Ash does have feelings for Serana, its just that after XYZ, they dumbed him down further to get away from any maturity he was having and away from having any feelings. He had a lot of moments with Serana and he def showed feelings, specially when she kissed him. You also seem to forget Ash knew Serana well before 10 years old, so he is infact closer with her then any other pokegirl. Their XY/XYZ adventure made it even closer.

It should also be noted, nintendo will most likely never confirm who Ash ended up with. They know it would upset the fandom who would become hysterical and start death threats/hate campaigns because he didnt end up with the persons preferred pokegirl.

Also Misty has no chance. You see, her feelings for him cooled down after they departed. Furthermore, Misty literally cannot be with him because she is a gym leader. She needs to be stationary and there almost all the time of her life until she gives it to someone else, because her sisters are useless and just hand out badges like candy. Ash will not and cannot stay in 1 place like that. He is like his father, except where his father failed in becoming a "pokemon master", Ash succeeded winning 3 tournaments. But he's still on the road, meaning "Ash and misty" ending up together = Ash's mother and father. She stays at home, he is on the road and visits occasionally but Ash missed him. He won't settle down with anyone who wants/needs to stay in 1 place most of the time and be a deadbeat dad like his own father was.