Rin and Seri, Inseparable

If the chosen target is an illegal target by the time Rin and Seri's last ability tries to resolve, the ability won't resolve. You won't gain life.

Rin and Seri's first two abilities don't trigger when you cast Rin and Seri.

An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger. It resolves even if that spell is countered.