You can have up to eight allies active in the final battle
against Meredith:
◆ Hawke and the three members of the active party,
plus Cullen: a mandatory fixture.
◆ Dog counts towards the total, so you might wish to
send him away in order to see other cameos.
◆ As Meredith’s health falls throughout the battle,
additional companions can join the fray. In order of
priority, these are Bethany, Carver, Varric, Anders,
Aveline, Fenris, Isabela and Sebastian.
Finally, there are three additional cameos that can occur
if you have completed specific quests: Donnic (if you
completed The Long Road and have Aveline in the
party), Zevran (A Murder of Crows) and Nathaniel
(Finding Nathaniel).

is there any difference in her involvement if you romanced her in DA1?

Actually orsino knows about that blood mage that kill hawke's mother.