"My relationship with my mother looks like it's OK but I don't know about the reality"
What phrase can I use (instead of that long sentence) to say that this is just from what it can be seen not the reality of it.
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If the speaker wished to suggest that their relationship with their mother looked OK (good, acceptable, healthy, etc) on the surface but might not be OK deeper down, then that person could say "My relationship with my mother looks OK on the surface". The word 'superficially' could be used instead of 'on the surface'.
Qualifiers such as "superficially" and "on the surface" all carry an implication of it actually not being so underneath.
If you want to avoid any such implied meaning, you would need to use additional qualifiers:
My relationship with my mother is at least superficially okay.
My relationship with my mother at least looks okay on the surface.
Using at least reduces the implication that it's necessarily the opposite in reality. (Because there could be more than just appearance.) However, the problem with both of those phrases is that your own doubt about the situation ("but I don't know about the reality") isn't implied.
Also, using such additional qualifiers doesn't meet your criterion of keeping the sentence short.
If you are really trying to express both outward appearance and some of your own doubt, I would suggest something different:
My relationship with my mother seems okay.
The word seems does double duty for both objective appearance and internal belief, and emphasizing it (either in text or verbally) acts to highlight your own uncertainty about the situation.
The word you're looking for is superficial
According to Oxford Dictionary
Not thorough, deep, or complete; cursory.
3.1 Lacking depth of character or understanding.
My relationship with my mother is superficial.
As pointed by @Michael Harvey, superficial may not convey your whole sentence. You may prefer to use façade (or any of its synonyms)
a false appearance that makes someone or something seem more pleasant or better than they really are
My relationship with my mother is pure façade.
EDITED: I've been criticized and downvoted but I still think that in most cases you just can say that a relation is superficial if such relation is good or friendly on the surface but you don't know really much about the other person's feelings or thoughts about you or about anything. Usually it's when the relation is superficially bad when you specify or add more content:
Superficially it seems that she hates me but I'm sure that she really loves me.