How to determine the equivalence point of a titration of a mixture.

Today, I'm graphing the titration curve for a mixture, to be precise, this particular case:


[![enter image description here](https://i.stack.imgur.com/NZTzZ.png)](https://i.stack.imgur.com/NZTzZ.png)


In order to get this done, i've been following wisely the mantra of "The strongest species (oxidizing and reducing agent determine the E(V)" and from there, I construct appropriately the Nerst Equation to suit the situation. This has given me positive results until the end point of Fe, where this doesn't seem to work for me. 


At this point, my logic is the following; we have Fe3+, Ce3+, and Sn4+, and we don't have any complete redox pair. Given this, the stronger oxidizer is Fe3+, and the strongest (and only) reducing agent is Ce3+; However, when I try to calculate the weighted average for the E(V) associated to these species, the value obtained (1.2) isn't anywhere near 1.05, my theoretical value, but closer to the upper limit of E(V). 


For me, it makes sense that it isn't 1.2V as there is another ion that lowers the potential, but I'm not really landing these ideas to determine the second end point nor the upper limit.


I'd appreciate any thoughts on this, thank you. 


