It seems that this "part-time" friend is just that. A true friend doesn't act like this. This is a friendship (and I use that term lightly) that was most likely forced due to the relationship of the husbands. No everyone's spouses get along and becaome friends.
If she only does things when she wants and not reciprocated, then I'd say stop considering her a friend - she is an accquaintance - nothing more. Friends don't trat each other like that.
Focus on your daughter and soon to be here new baby son look into play groups. You'd be amazed at the real friends you can make that way. Expand your friend network!
The only other thing is that maybe she has something going on in her life that is stifling her. Not all husbands are 50/50 and she may have no say in what they do when he is around, maybe she has personal or financial problems. Stress is a big factor in the way a person acts. Ask her if something is wrong, if she needs help with something, maybe she just needs a break from the kids herself. Try getting a sitter for a couple of hours and go get a pedicure together. All you can do is ask, if she says nothing is wrong, then leave it alone and move on.
V.