I may be the only person here to defend pearl or negai but here goes nothing.
For pearl I don't really see it as a big brother/little sister (that's more advance turf) but more like a boy and girl best friend. While the Hi 5s in and of themselves would not be real shipping hints its a sign of closeness between the two no other girl could claim and the fact that the characters are all 10 kind of shoots down kissing or romantic hugging etc. I also get the feeling that while Ash Misty and Brock was a unit. Ash May Max and Brock was one unit. And Ash Iris and Cilan were all one unit its hard to think of one without the others. But with the DP group I honestly felt that asides from the doc brock filler Dawn and Ash could honestly just travel together alone with no issues unlike With misty where they would fight constantly and would probably turn ugly if not for brock or may where Max and Brock were kind of needed as connectors joining the group. The two also connected on an emotional level with Ash's concern for her after the solaceon loss and Dawns little performance after lake acuity shows that they cared for each other's wellbeing and happiness. A few other hints would be the cheerleading outfit Ash's Increased interest in contests which is not something I bet either of them would do for just anyone.
Another interesting aspect of pearl would be their startlingly similar personalities. Yes there is the dress up and the boneheaded stupidity ash can often show. But they have similar outlooks on life and always tries to put their best foot forward. Irl relationships operate much the same way ie opposites rare attract. I also see Lyra and her involvement as pearlsbipping evidence. She saw the attraction between the two and then when dawn had a rather suspiciously quick denial Lyra moved in on Ash trying to set her up with him. Eventually she realized the relationship was too strong for an outsider to break and tried to model her and kory's relationship on theirs(this coming from a true shipper on deck). For that matter last thing I'll say on pearl is that Ash seems to care about dawn's opinion of him a lot. When iris calls him a kid he just shrugs it off when dawn does it he feels devastated and betrayed.
Now for Negai shipping I can see it happening as well and for more reasons than just because Iris is there. From the first episode Iris saw something in Ash besides just pity or scorn as she could have just pointed out the directions and sent him on his merry way. Then she actually helped support him at the first gym despite constantly belittling him which I consider a way for her to cover up her own insecurities rather than of a genuine desire to hurt. Iris chose to follow him not out of vengeance for a bike or to spend more time with Pikachu but she sincerely wants to travel for months with asttanger she barely knew without an ulterior motive. Iris also while rightfully berates ash for his stupidity still recognizes his moments of clarity and never moves into physical abuse. She also cheers for ash the hardest of any pokegirl with the roxie battle and while the last MC pokemon ship lagomorph failed oshawottxemolga looks far more likely as both parties showed Intrst in one another and are open to romance. Cilan has even noticed there is something between the two. While this ship is lacking slightly In quantity Dur to BW still running it isn't lacking In quality plus ash crossdressed for her.
As for why I support these over poke and advanced. Ash has never shown any real attraction towards misty's crush which wasn't really that great in the first place and was a case of being the first more than anything. Plus 4kids blew it out of porportion. Until the last episode ash thought of misty as a friend who followed him for bike money until she ran away In tears. Their interaction ended there with no real references later on. Eventually I got the feeling that she got over him I. Chronicles spending a lot of time with Tracy and ash just moved on holding the lure to remind himself of the good memories rather than misty herself. As for may I see it as a student teacher for more than the others as even at the kanto GF her style is still taken completely from ash. Plus drew's existence shoots down advanced worse than any other competing ship for the other top 3. I'm not that big a contest shipper but I do see the attraction and pair thn together.
Tl;Dr don't discount pearl and negai just because you already have an OTP. While you may not consider then strong enough or already have an OTP they exist and are just as valid as any other het ash ship.