These raise a couple of interesting issues. Firstly, given the general consistency I'm sure that most of us would agree that these are probably best understood to be connected products of the same scribal activity. Secondly, given that both 1 Peter and 1 John are introduced as 'the first epistle of Peter' and 'the first epistle of John' respectively, it seems likely that the meaning of the others is thought to be consistent, i.e. 'the second [epistle] of Peter', 'the second [epistle] of John', 'the third [epistle] of John'.
So now to the ECM2. Firstly it attributes the inscriptions for 1 Peter (p. 103), 1 John (p. 263), 2 John (p. 369), 3 John (p. 387) and Jude (p. 403) to 01C2 (in each case 01* is given for "om.", i.e. originally lacking any inscription); but the inscription for 2 Peter is attributed directly to 01 (p. 203). From this we would deduce that Sinaiticus originally had an inscription for 2 Peter, but nothing for the other Catholic Epistles until the seventh century. But this is clearly wrong, both because it is inconsistent, and because there does not appear to be a clear link with the C2 corrector. Probably within the parameters available in the ECM2 they should all have been attributed to 01C1 [covering corrections from the fourth to the sixth century]. But in any case the Sinaiticus Project attributes all these titles to S1 - a scriptorium corrector, and this seems right to me. This means that they could be regarded as an aspect of the original production of the manuscript as a whole.
For these I would suggest that we regard them as an aspect of the original production of the manuscript as a whole, and hence simply 01. Or perhaps if we were being pedantic we could attribute them to 01Z(S1) - an addition (Zusatz) by a different but contemporary hand (and not to give any 01*).