There are three primary ways to win with tokens: You attack with enough creature tokens that your opponent cant possibly hope to block them all, You pair your token generation with Mirkwood Bats, Impact Tremors, Zulaport Cutthroat, and similar effects that drain your opponents life total as your tokens enter and leave the battlefield, or You have sacrifice outlets that let you cash in tokens to kill blockers and/or drain your opponents life, like Goblin Bombardment
Creatures that come under your control after it resolves will not have the bonus
Id be looking to get in on these under 0.20 in bulk, with a view to buylist them to CardKingdom or similar a year or so down the road
But you can't use proxies in official competitions and events
Especially because it even gains the ability to tap for mana, but this wont amount to much more than a hard-to-use 3 mana mana rock, and thats not worth a card in Limited
There are some solid card advantage engines that can take advantage of the soul sister abilities