First, a Word to the Wise: Never call a Yuan-ti useless. They spit poison, you know.
I'm probably preaching to the choir with the ideas below, because you've already been thinking about this longer than I have.
Looking at snakes as a general category, you've got your biters and your constrictors.
By choosing Dex over Str as your primary combat ability, you've opted for biting, finesse melee weapons, and ranged attacks, all in keeping with the preferences of a sorcerer, and good enough for the somewhat more melee-capable warlock.
Two of your ASIs should, of course, eventually go to Cha to get that up to 20. Your concern about the race's favored abilities being "useless" (I'm not totally sure what you mean by that) is just something to challenge your imaginations; that boost to Charisma is certainly not useless unless you really do not want to be playing any Charisma-blessed class in the first place. Intelligence less so, although that is a real leaning for the overall race itself.
As AL adventures generally don't go all the way up to level 20, my own inclination would be to improve Dexterity for over-all optimization. If feats are allowed, take Resilient to boost Dex to an even number and gain proficiency on your saving throws. Proficiency in Dexterity is the only ability that can affect attacks, defense, some saving throws, and initiative.
If not taking Resilient for Dexterity, taking that feat for Constitution to up your concentration saving throws is your next best bet, with the following ASI giving +1 each to Con and Dex. Of course, you'll need to survive to 16th level to get that boost, and an AL campaign might not get you there.
Since you're playing siblings, you might want to consider choosing the same background to tie your PCs together, but that's not an over-riding thing.
Given the status of purebloods in Yuan-ti culture, I think of them as middle or working class. For the paladin/warlock, either the Acolyte (patron) or the Soldier backgrounds seem appropriate. For the sorcerer/warlock, the Acolyte also works for the same reason, but having one of these PCs gain contacts within either the criminal world or merchant classes would add good role-playing options in light of Volo's information. If both PCs have similarly low Intelligence scores, there's no point in the Sage, but that's the only background I'd rule out.
What sub-classes to take for a sorcerer/warlock?
The reptilian connection has Draconic Bloodline written all over it, and the common link of poison with the green dragon suggests that particular choice.
Snakes can be creepy; I'd favor the Great Old One (for that alien race feel) over the Archfey (my second choice, since snakes are part of the natural world, even if the fey are kind of ultra-natural) when selecting a patron. If either of these warlock's doesn't go for the blade pact, it should be this one, and that would be to choose the tome for those extra cantrips from any class. More ranged at-will casting? Yes, big time.
Sub-classes for the paladin/Warlock?
Because of the paladin's combative aspects, the warlock's Pact of the Blade makes the most cross-class sense. You probably had that figured out already. The Great Old One, Dendar the Night Serpent, may be both the most likely patron to a Yuan-ti and the most worrisome. Yikes! For my peace of mind (it's disturbing when my dice bag begins to slither across the table), please choose a relatively "nice" deity such as Merrshaulk or Sseth, or create your own personal patron from Volo's information about lesser serpent gods. I like the idea of the paladin's Oath of the Crown, modified to be specific to an oath sworn to the Yuan-ti domain itself (adapt Oath of the Crown from the Sword Coast book?), re-fluffed to whichever patron you select. From the same sourcebook, perhaps renaming the Order of the Gilded Eye to the Order of the Slitted Eye?
Getting back to the abilities question, for the paladin/warlock, favoring Strength is pretty much required. Think in terms of flavoring it toward constrictor snakes, maybe taking the Grappler feat early on.
I hope these suggestions are helpful.
