If Ian is in love with Lena, he can't commit to Holly.
I don't remember if Lena being in love with Ian stops Ian and Holly from commiting to each other (don't think so but might delay it a couple scenes while Ian reassures Holly past the drama).
If they are both casual about sleeping with each other (or not sleeping with each other at all) it should be completely safe. Honestly if I'm doing commited IanxHolly I just keep things platonic between Ian and Lena, avoids so much unnecessary drama.
Holding out hope that there will be an alternative entrance to the "throuple" route later with Ian and Holly in a relationship and Lena being the one inserted into it because she keeps having the hots for Holly, but it looks like the "throuple" route is mostly being designed for the high-school tier drama of them both having kissed Holly while dating each other and taking 10 chapters to resolve to just fuck her together.