Not sure if I remember correctly, but somebody (on either site), who is master carpenter was saying, that it should be done with long pieces of wood (maybe plywood or such, don't remember...). They should be fitting tightly within beam's indentations on both sides, and should be glued-in and screwed-in.
I guess you could also install cross braces (right angle to I beams), to prevent them from twisting.
Unless some pro chips-in (start another thread with more appropriate title - like "how to hang PJ off the I-beam"), maybe you should consult actual carpenter?
I have no idea how big is the risk of beam's failure if you don't reinforce it, or do it incorrectly (for starters, you would have to know both it's theoretical limit, and actual load), but those PJs ARE heavy, and best seat is almost exactly under the beast
I made sure I could hang off EACH of mounting points (~180lb), but mine are regular beams... They didn't budge at all.
In case you want to try NOT to reinforce those beams, remember being courteous to your mother-in-law can pay back big time