By blood and common law yes, completely. As for being behind the Yorkists, if you by pure primogeniture they would be behind them but legally speaking the whole Lancastrian claim was based on the idea parliament could change that so they would be put above the Yorks, as per the decision of the Parliament of 1399. Moreover, you can always resort to the good old having attainders being passed against them.
The one claimant who might
maybe go for the Lancastrian succession over them would be the Marquess of Dorset, as descendant of Elizabeth of Lancaster. That one would essentially depend which one would prevail between children who where born legitimate get priority over those legitimated by subsequent marriage vs sons come before daughters in the succession order. I admit I don't know
that much on 15th century English Common law