What do I do with these awful IVs units?

Ninian is a dancer anyways, so you won't be doing much attacking with her in the first place, so -Atk isn't so crippling.
Bride Sanaki hoever still has a relatively high base atk of 33 even with a bane, and considering her optimal set is the Gronblade set (IMO at least) it isn't that bad as long as you have a hone fliers or other buffer around, and it will give her enough dmg to take out most units.

Ninian. Other than blade....shes just not gonna be even a good red check. So just double down on support build imo. Could go TA for SOME offensive presence (swordbreaker if you REALLY want her to check them).
Also fury or darting stance for the rare times shes left out in the open and has to take a hit

For sanaki. Again blade is an option.

For her original tome you could go with her budget flier set. But with qr in seal slot. This is so that she can set up on mages thanks to her high res and good spd with flier emblem

Ninian's a dancer and her attack is already pretty awful from the start, so it's okay, imo dancers should try to not be easily killed and with a res+ she'll manage to withstand some powerful mages, so if you're in chain challenges, you won't lose a dancer which is very beneficial. Just slap swordbreaker onto her if you want her as an offensive unit

Sanaki though, she can most likely depend on flier buffs, or sacred seals. It's best if you place her in a flier team with a lot of buffs