We are finishing our attic in a 1911 house. The current joists are true 2"x6" with 10'-12' span. We are sistering them with 2"x6" LVLs. We will be adding a bathroom (shower, toilet and sink) and the plumber wasn't sure if drilling through the sistered joists was allowed. Any thoughts? Would we still follow the 1/3 rule, so nothing bigger than 2" holes for plumbing ?