<grinning> My first reaction was: It's only 2 half days a week.
The second reaction: What a great time to start... To give her six whole months of continuity before her sibling comes home. That way, the preschool will already be set into her schema of what's normal & fun... and doesn't get jumbled together with baby craziness.
Of course, these are my opinions and reactions. And here are my two bias:
1) We had a FANTASTIC preschool experience. We started off slow, too. 3 half days per week. About six months later we took the "lunch option", which added an hour (because he sooooo wanted the extra time, and low an behold, I managed to "fill" that extra hour to myself ;). The next year we bumped it up to 4 half days. The plan* was that that would lead naturally into 5 half days for Kindergarten, which would lead into 5 full days of first grade. I have to tell you, I did NOT sleep the entire night before his first day. I was soooo nervous. Him: Fine. Me: A Wreck Pretending Not To Be. I could have gotten an oscar nod for that performance. Oi. But he had the most WONDERFUL 2 1/2 years at Chelsea House. If we ever make it "big", or win the lotto I'm going to be making some huge cash donations to that school. They were such an amazingly good influence on my son, I not only wish he could still be there, I'd like to set up a grant program for other kids to go there. Have I mentioned that this bias is that I LOVED our preschool experience? ;)
* See below
2) We homeschool. Our plan did NOT go as scheduled. First off: by the end of his last year at preschool he was doing 5 hour stretches... but the kindergarten 1/2 day was only 3 hours long. Ummmm.... & the full day was 8 hours. HUH. Not a lot of give, there. So we decided to go full time, rather then backwards and then thunk him into reeeally long days the next year. The 2nd problem was that my son LOVED school (montessori preschool), until he started at public school. Then he started to HATE it. Crying literally everyday as soon as he got in the house. He was bored. He was t.i.r.e.d. He liked his friends... but the school itself was miserable. The curriculum at his proper school wouldn't catch up to what they had ALREADY learned in preschool until the 2nd/3rd grade. Long story short: For 1st grade, this year, we decided to homeschool. So, just because your smart little girl heads off to preschool, doesn't mean your plans might not change midstream, if you end up like us, you could very well be with her 24/7 again for the next 10 years. :) (Most homeschoolers start going to college around 14-16)
Now, my assumption is that since you're a psychologist (or in the process of?), is that you practice authoritative parenting? With gradually increasing areas of independence and responsibility? If so, your plan sounds perfect, If I were you I would enroll her and keep an open mind. See how it goes. See if she's thriving and you're thrilled. Introduce, observe, plan, & act... instead of a preemptive strike. ESPECIALLY if the preemptive is brought on by a wave of self doubt introduced by your friend coupled with the oh-soooooo-frustratingly-natural "My baby's growing up!!!" pangs we all get.
Anyhow. Just my opinion. Choose what's best for you and YOUR family. We're all so very different. Good Luck.