Pivi:
Hehe, because there's nearly no romance in AC. Nor between Cloud x Tifa, and nor between Cloud x Aerith.
Nothing romantic, but there was certainly much affection (deep warmth) portrayed. As far as affection goes, I saw much more between Cloud and Aerith. He appeared closer to her than anyone else, including Tifa.

Yes, because Cloud did not experienced a peaceful life like that before, he was anxious, and he did not know how to live a life like this. But like everything, with time he will come comfortable with it.
When someone experience something new, then he won't do very good at the first time, but at the second, third, ... he will get used to it.
Actually, I think silentchild was trying to point out that Cloud even paid more attention to the children than he did to Tifa. It's why she was a bit envious. It's a very good point because it indicates that there isn't any romantic feelings being reciprocated by Cloud. If there was, Cloud would be paying attention to her, and thus there would be no need for jealousy.

I just suggested, that I think Cloud considers Aerith as part of his big family (his friends), and not as part of his small family (Tifa and the children).
Well, first off...if you're going to separate Cloud's family into two categories, you can't forget poor old Barret. I think he fits in with Cloud's "small family" as well, no? According to CoT, he, Cloud, and Tifa came together to form this "family", didn't they? So the guy wasn't present when the picture was taken, but you certainly can't exclude someone just because they're not in a picture. As Anastar has also pointed out, Tifa considers their family as friends coming together, not a romance thing where she and Cloud tied the knot.

Secondly, the small portrait was behind the big portrait and off to the side. It wasn't exactly the focus of the camera either, as if indicating that Cloud's real completed family actually includes everyone as shown, and not just him, Tifa, and the children as Clotis previously thought.

His family includes all those who fought with him to save the Planet, the Avalanche companions you could say. However, Aerith wasn't even in the pictures, so I'm not sure how you could have lumped her in with the "bigger family" rather than the loveY doveY "little one" off to the side consisting of just Cloud, Tifa, and the two children (Barret as well). Personally, I think she has her own little category that does not include anyone else as I believe she has a special place in Cloud's heart.