I'm not sure whether the relationships are so complicated or whether i misunderstood.
So the case is as follows the supposed bridegroom and the sister of the supposed bride have the same foster mother who's the sister of the father of the supposed bridegroom! And both sisters are the daughters of the man's paternal aunt!
Whom will be a mahram by breast-feeding
Well among those who are prohibited for marriage quoted in the Verse (4:23) you'll find also:
... your [milk] mothers who nursed you, your sisters through nursing...
Assuming the breast-feeding was counted: Then from that point of View scholars would tell you that the mothers of the foster-Mother (whom are considered as her roots), her daughters (who are considered as her branches), aunts and sisters would be mahram for him (You can read about it here in Arabic and read for example in Sahih Muslim 1 & 2), while her sisters or aunts wouldn't be mahram (as they are only her surrounds).
So according this as it's not the "real" mother of the supposed bride whom is his foster mother, then he can marry her as she isn't a mahram as a (surround of the foster-sister) (see also this Fatwa in Arabic and this Fatwa in English). But if the foster mother is the real mother of the supposed bride, all depends on how one suckling would be interpreted. About relevant sucklings read the following:
When one would be considered as breast-fed
There's a Hadith Sahih Muslim, see also .
You could read the details in my answer about the interpretation of this Hadith. There i quoted that there one can find that most scholars say that this Hadith is an ahaad and it couldn't be taken as an evidence for naskh in the Qur'an so most scholars would say that even one suckling is enough to be counted as breast-feeding, while Shafi'i's took the Hadith as is and said at least a couple of suckling if not 5 would be counted as breast-feeding (apparently this is also the opinion of later scholars like the Hanbali school). But I'm not a scholar and only quoting what i found, so you should ask somebody who knows it well!
If it was only one suckling shafi'i and hanbali would say that the supposed foster mother isn't such and therefore her children won't be mahram. If it counted as the maliki madhab and earlier scholars say that the girl would be a mahram!
And Allah knows best!