It isn't so much that she took over their spheres, as that every sphere has always overlapped slightly with other spheres. I tend to think that Sirona just had to emphasize the aspects of her sphere that are closer to those of the fallen gods, since her sphere just happened to overlap most with the fallen spheres.
I tend think that Sirona's sphere overlaps with every evil god's sphere except for Mulcarns. Mulcarn was the least evil of the evil gods, so despite opposing The One (because his sphere naturally opposed the change of loosing the power of creation) his sphere wasn't really corrupted, and he continued to fill the role for which he was created. His sphere became a little harsher, but he still played an important role in helping Creation rest.
Mulcarn's sphere would overlap with Danalin's (whose sphere would also overlap with Sucellus and Sirona) and Arawn's. These neutral gods probably acted to keep rest what it was meant to be, but now both of them have given up.
It is known that before his fall Sirona was the closest to Agares, and that she is now the only one who thinks he can still be redeemed. The overlap between Hope and Spirit is quite strong, as you could see from the spirit II spell. I think Creation also overlaps a little with hope, so Amathaon probably helps a little too. (I tend to think that Ceridwen's sphere overlaps rather strongly with Agares' too, and keeps him from repenting.)
Ceridwen's sphere now focuses on unhealthy relationships, so other gods would have to intervene to keep relationships healthy. Ceridwen's and Sirona's spheres both deal heavily with caring about those close to you. The Dimensional sphere would just be about caring for with whom you have a relationship, for the sake of the relationship. Sirona's is broader, so it would include caring for those close to you but also for those you just met or will never meet. Junil's sphere also overlaps with Ceridwen's, as both deel with relationships based on hierarchical bonds and adhering to someone without the need to actually love them. Nantosuelta's sphere has always been the exact opposite of Ceridwen's, but she too is important to relationships because she deals with making us value each other's intrinsic qualities and making relationship healthy instead of obsessive.
As the Spirit III spell indicates, Spirit overlaps with Trust. I tend to think that Water and Trust overlap too, as water is the sphere of serenity and being comfortable with those you know. It is not a passionate love, but the trusting live between old friends. Of course, Danalin's dreams mess this up. Sun would also be important to trust, as those dominated by Sun are extremely honest. Dagda may be involved a little too, as you can trust force to be unbiased and not betray you without permission.
Foresight was supposed to find the best outcomes for everyone, as spirit/wisdom still does. It would overlap more with Sun/Truth, however, as both are about finding things out and revealing how things will probably go. It would overlap somewhat with Metamagic/Knowledge, and Danalin's aspect of gradual revelation based on experience and reflection instead of epiphanies like Lugus's sphere. In the last 2 cases, the overlap was enough to allow Mammon's forces to break into the spheres of Oghma and Danalin and begin to conquer them. He has been filling their precepts (poorly, so as to corrupt them) as much as they have filled his.
Aeron, angel of strength, was supposed to be about the strong protecting the weak. This is very much within Sirona's sphere now, except those who do the protecting often don't really have the strength to do so. Valor would probably be in this sphere had it not been corrupted, so Junil has been very important in filling this gap. Bhall was probably even more important in this regard until she fell. Since his resurrection, Sucellus has been very important in healing the weak and giving them the strength to fight on, even after death. Had Basium been made God of Life he would have probably made up for the lack of true strength even better.
In Camulos's dereliction of duty, Sirona has been very important to maintaining peace. Her people are (until Ethne's time at least) the most pacifistic, and the most trusted mediators in peace treaties. Lugus and Dagda are also very important in mediation and setting strife. Junil would be somewhat involved too, but his methods are often too harsh and his peace rather contrived.
Bhall is still the goddess of fire, but more than that she is the goddess of passion. Before her fall, this essentially meant Righteous Indignation. Now, it is just fury. She would have helped compensate for Aeron's loss before her fall, but since then she and Aeron (and Camulos) are close allies. Sirona probably tried to keep people in good enough spirits to continue on without the drive of her passion, but doesn't do a great job of it. Junil was very close to Bhall before her fall, and has largely stepped in to fill her role. Holy warriors now tend to be driven more by duty than by passion, but the effect is largely the same. The return of the resiliency Sphere of Life would help make up for the loss of the determination of the sphere of Fire.
