I think bringing up The Calling brings up some timeline inconsistencies between book and game.
In the game, it's explained that one huge reason Maric didn't openly claim Alistair (besides challenging Cailan's claim to the throne) was because it would humiliate his wife. More than one character in the game mentions how he (apparently) conceived Alistair while married to Rowan.
This makes sense, as this was back when the series was more directly following one of its inspiration series A Song of Ice and Fire (or A Game of Thrones for the TV show). Alistair's situation is very much like that of Jon Snow--a noble bastard who never knew his mother, joined the Grey Wardens Night Watch, then was offered the crown at one point. Loghain mentions (if you spare him) how Maric wanted to acknowledge Alistair, and almost did, but the only thing holding him back was how it would humiliate Rowan; the wife he cheated on to conceive this bastard who, if acknowledged, would compete for her legitimate son's inheritance. This is almost surely a direct reference to Jon's father Eddard Stark acknowledging him and raising him alongside his legitimate children, much to his wife Caitlyn's utter disgust and degradation. Caitlyn loathed having Jon around (before she had him shipped away) because seeing him was a constant reminder of her husband's infidelity, and made her feel threatened that he might take her kids' inheritance.
Problem is, in The Calling Rowan had been dead for a few years by the time Alistair was born. There was no tawdry infidelity involved in his conception, no wounded pride or hurt feelings from his wife if he took him in, no angry step-mother to loath her husband's illegitimate son, etc. Suddenly, that noble excuse is OUT.
Personally, I hold Eamon more accountable than Maric. You know your wife is mistreating a child put into your care. STAND UP TO HER. I myself was mistreated by a mean, petty, spiteful step-mother who was jealous of the fact that my sibling and I were her husband's kids from a previous marriage, and spoiled her own kids (my half-siblings) rotten while trying to make us feel as miserable as possible. And few things made me ANGRIER than my father just sitting back and letting it happen. He knew she was treating us like dirt and just pretended not to notice, took her side in every argument, would tell us in private afterwards that we just needed to give her her way to "get along," etc. I have no respect or sympathy for Eamon.
However, Eamon and Isolde's mistreatment of Alistair doesn't completely excuse Maric in my book either. While you could almost, sorta, maybe argue that Alistair isn't really anything to Eamon--the illegitimate child of the man who married to his sister, whom he either cheated on to make the child or she's been dead for years--but for Maric, Alistair is his kid. If you know your kid is being abused in the home you put him in, when the whole reason you put him in that home was to give him a better life than the one you think you could give him grooming him to be a prince in a palace. You conceived him through fairly legitimate means since you were an unmarried widower when you conceived him, and have no current wife or adult relatives who'd feel threatened by him. What have you really got to lose by pulling your kid out of that abusive environment.
Yeah, yeah, "threat to Cailan's rule," "political instability," etc. It's just... ugh. Whatever.
Many parties are at fault as far as I'm concerned.