I agree with Callaway with the kids rather being a duo than having a third party around thing. Ive seen it with boys as well as girls.
Unfortunately its common Mcjagger, and you cant really force kids to want to play with others, the girl down the street is obviously being cruel in the sense that she and your youngest exclude the oldest (even though they probably dont understand the true consequences of their actions in the sense of how it makes your eldest feel, but they will all learn soon enough through experience no doubt).
However you cant always expect one of your daughters friends to want to play with the other sibling. If your youngest likes the girl down the road and they both play well together (and she isnt devious in any other sense) then its a shame to break up their friendship, maybe you need to try help your eldest make some friends, the soccer team could be potentially great for that but you need to bare in mind that kids just like adults build friendships and bond or identify with one person more than another, only kids find it easier to exclude the parties they dont bond with or want around and are less tactful about it.
Talk to your youngest, explain how her sister feels and talk to her about sister solidarity and how they should try stick together if you must, and even get your partner to speak to both girls as in your youngest and her friend together, the best way to get through to them would be to explain the situation, how your eldest feels, then ask them how would they feel if they were excluded and kids said the same things to them such as "we dont want to play with you" or run away etc, wouldnt it make them sad, because it could happen to them at some point and its not nice. Suggest they try make a little effort before deciding to exclude her, even ask why they dont want to play with the eldest, see if theres an actual reason (because sometimes there can be even though it can be a silly reason, but it can help you sort things).
Its important to explain to your eldest that these things happen to most kids sometimes even adults and not to take it personally or to heart, some people find it easier to be in a coupling than in a group, also that even though some people are just plain silly or cruel, that more commonly alot of the time its more to do with people just making friends easier with one person than another weither it be because they have more in common or whatever the reason may be, which she will find out more about as she gets older and picks her friends and companions etc.
I hope the soccer team goes well, and both your daughters make some decent friends out of it.