بسم الله الرحمن الرحيم
In the above mentioned case you cannot marry that girl as both of you have turned foster brother and sister with the relation of suckling. As the nikah of real brother and sister is unlawful, in the same way the nikah of foster brother and sister is unlawful. The holy Prophet Muhammad (صلى الله عليه وسلم) said:
يحرم من الرضاع ما يحرم من النسب.
Allah knows Best!
Darul Uloom Deoband