5th dnd – Can you cast a spell on someone in the ethereal plane, if you are materially and the Real Light spell is active?

Ethereal said:

You do not know all the objects and effects that are not on the ethereal plane

Unless the caster has the means to carry his effect in the ethereal plane, all the spell effects he creates will be in the material plane. And since a creature under the effects of ethereal is immune to effects that do not occur in the ethereal plane.

The additional support comes from the part you have already mentioned:

Creatures that are not on the ethereal plane can not perceive you and can not interact with youunless a special ability or magic has given them the ability

See true could give a creature the ability to see you, but it does not magically give them the opportunity to interact with you. You are still in a separate plane. Thus, they are not able to target you with effects for this reason because it is a type of interaction.

Spell that target at sight still need to meet other requirements

As a kinship: when a spell says you must be able to see the target, it does not mean that it is the only condition required by the spell to be able to target something. For example, all spells must have a clear path to the target (unless otherwise specified in the spell effect) by default.

Although nothing says it explicitly in the rules, it seems that the most logical decision would be to not clearly define the path that leads to things separate from yourself. Spell effects can not cross planar boundaries unless they say so explicitly. The addition of a requirement that you must be able to see the target does not circumvent these other considerations.