First it's my respected ukhti,
Then it's ukhti that I really like spending time chatting with,
Then it's ukhti who I'm beginning to develop strong feelings for,
Then it's getting to be real heart-warming every time I talk to her,
Then it's ukhti I "love you for the sake of Allah",
Then we're both really losing composure and respectability,
Then it's lucky if we're seperated by a few oceans......
There comes a time when both realise they have feelings for each other, that's when they have to put the brakes on and start respectable fiance mode, if they feel they can't live without each other, tell mum and dad to be lenient in compatibility comparison.
Also explain the genetic benefits of marrying a little far due to a good inheritence of diversity and survival qualities (won't apply in this case since they're foster).
Trying to pull them apart from each other won't usually help if they've developed deep feelings for each other since the sis would be depressed and hate you, and both you cousins will probably develop a sense of hatred, anger, or dislike.
So see where it's at, and try and bring it back into Allah's limits asap. Don't let your ego get in the way - it may end up injured worse later if you're oppressive.
https://en.m.wikipedia.org/wiki/Fakhitah_bint_Abi_Talib
You can get informed by reading up, though Allah :swt: knows best the reason why the messenger of Allah :saws: later refused to marry her, in other places, it is reported that he :saws: said that he felt no desire/need to marry her when she came back and offered herself in marriage. And he :saws: was human.