Mine are 11 months apart, DS adjusted really well. My husband took over bedtime with him for a while and paid him extra attention. Now he occasionally comes over and tickles her or gives her a kiss, but then goes on his way and doesn't bother with her. I would say it has been a very easy change.
Re: 12.5 months apart
That's our exact spacing-their bdays are three weeks apart.
my ds was pretty blissfully ignorant to his sister coming home. We had no difficulties with him at all.
Mine are 17 months apart but even at that age, DS1 didn't really get what was happening and never had jealously issues.
I think it's one of the pluses of having kids close together, they are SO adaptable at that age!