No matter which option you choose (with Gascard's help or without), there is no way to save your mother from her unfortunate fate. Before Leandra dies, she tells Hawke that she is proud of them. ... Hawke can choose whether to reveal the manner of Leandra's death or not.

Is gascard DuPuis the killer?

Gascard DuPuis is dead

As he couldn't have been responsible for the shade attack on Emeric, the conclusion is that he was not the killer; the only storyline impact is the restriction imposed in the later quest, All That Remains.

Who should I help First Isabela or Aveline?

If Isabela is present but you choose to help Aveline first, you immediately activate this quest – this will cause Isabela to leave permanently. If you don't want to lose her, help Isabela first (the To Catch a Thief quest), after which this quest will continue.