When twins stop being best friends

Has anyone dealt with twins who have a "break-up"?  My girls, who will be 4 in April, have always been best friends - the typical twin package deal.  Now, Twin A seems to have moved on.  Twin B, who is the more sensitive one, is really struggling to deal with it.  I am not sure what has happened.    Maybe it is that they are finally in preschool and outgoing Twin A is making new equally outgoing friends; whereas reserved Twin B has befriended the girl in class who literally does not speak and prefers to communicate with faces and gestures.  Maybe it is because their little brother, 19 mos., is getting to that fun independently playful stage and he has become an alternative playmate.  Maybe our efforts to foster their independence have backfired (e.g., they have their own rooms, we routinely engineer situations where one twin is the only child in the setting, we often say to them "ok, that's what your sister wants, but what do you want?").  However, determining the cause of the problem may not be as immediately important as dealing with the end result - a lonely Twin B who is very emotional and reacting out of those emptions, thereby causing others to react emotionally.  
 
For example, this morning Twin B was literally chasing after Twin A screaming and crying, begging Twin A to play Candy Land with her. This resulted in Twin A running away from her sister shrieking "NO!!!!"  We solved the problem by having Twin A play the game in one room while Twin B and Brother colored with markers in the kitchen.  Scenarios like this keep playing out and we are constantly in defensive damage control mode.
 
We need to get the situation under control because it is contributing to other issues, such as the recent increase in Twin B acting out to get our attention.  She seems to be taking the approach that any attention from anyone is good attention, and we all know the problems with this.  I think forcing Twin A to play with her sister will backfire, so I am thinking the approach is to work with Twin B to accept the situation.  But I am not sure.  Anyone have experience with similar situations or have any suggestions on how to help Twin B cope?
 
HELP!

 

Siblings Without Rivalry is an excellent resource for dealing with all things sibling related.

I'm also a big fan of "sportscasting" and letting them solve the problem on their own. For example, based on the Candyland situation I might have said "Twin A is playing Candyland. Twin B would also like to play. It seems like Twin A would rather play on her own. I wonder how we could solve this problem?" Then wait (this is the hard part). They will likely continue to argue/fight/scream/cry so you continue sportscasting (sometimes neutralizing the contentious item by putting your hand on it gently but firmly). "We need to discuss this and reach a solution. I will hold the game until we have made a decision." Wait some more. If it seems like they're arguing in circles, reflect back to them what you're hearing "Twin A wants to play by herself because she feels Twin B always knocks the board over. Twin B feels playing by herself is boring." And so on. They will eventually reach their own resolution and this is another tricky part because it may not always seem "fair" to us but assuming they are both content with it then it's best to let it be. Yes, this is a time and attention heavy approach but the more you use the technique the less involved you have to be until eventually they do it all themselves.