A stepparent who sits in the back so your dad can have the front row is showing love that doesn’t need credit.

Giving up the good seat is a decision made quietly, with the full knowledge that nobody will mention it. Love that does not require acknowledgement is rare in any form, and it is particularly rare in a situation where somebody could reasonably be asking for recognition. What you are seeing is restraint, which is harder than enthusiasm.

You could say something about it, once, in a sentence that does not make a big occasion of it. Noticing is most of the gift. If saying it directly feels like too much, telling your dad what you noticed will reach them anyway.