While Healer's Staff certainly carries bonuses of 20%, the maximum MAG stat on a Nirvana built from a Healer's Staff is 424 comopared to Hope's best weapon which carries 917. With this in mind, combined with the 1700/1900 split on base magic from the crystarium...

Vanille: 1700 (Crystarium) + 424 (Healer's Staff upgraded to Nirvana) = 2124 Magic + 20% = 2548.8 Magic
Hope: 1900 (Crystarium) + 917 (best magic weapon upgraded to Nue) = 2817 Magic

It would then come down to exactly how the magic stat factored into healing, and if that 20% is based on magic alone or is actually adding to heals, etc. Either way, just because she has a 20% bonus on that weapon doesn't mean she's instantly the best healer. I mean, you could get a 20% defence bonus on a weapon for Hope but it won't make him the best Sentinel.

Once you come down to it, while Vanille potentially could be the best healer, it's really not as cut and dry as you might think and saying that it's sexism is something I would call out as wrong in this case. Hope is actually identified by SE as the better healer in the official guide, where it notes that Vanille's advantage over him as a medic comes from her additional HP. Vanille's primary role in the long run is, for me, Saboteur.