I don't have a regulatory answer, but we (the bank) are a subsidiary of an insurance company and they maintain our servers and e-mail services.
We just went through an IT exam and the OTS didn't have any issues with the arrangement. The did visit our parent and conduct an exam looking at all of the thing they exam us for, but otherwise it was no different that any other IT exam.
I'd contact your regulator and see what they have to say. In our case, there is significant cost savings (even after paying an IT allocation to our parent - an affiliate transaction) and so they don't seem to have an issue.
Good Luck