If I had a daughter who (is/were/was) cute, I would be very happy.
(Written to mean that I have never had a cute daughter, so I would be very happy if I could have one)
I think "were" or "was" is grammatically correct because the sentence is of a hypothetical situation. So, "is" seems to be grammatically wrong to use there, and I know a more natural wording would be just "If I had a cute daughter", but I wrote it like that on purpose to ask this question.
Best Answer
UPDATE
I'm no longer sure about my answer. I have been thinking about this over and over in the past few days but cannot reach a definite conclusion for myself. Hence, 'was' instead of 'were' might be the correcter option for the singular. This was also pointed out by @alephzero, here below in the comments.
ORIGINAL POST
This is the subjunctive: 'were' is correct and used in either the singular and the plural forms. Hence, in your specific example:
Singular: If I had a daughter who were cute, I would be very happy.
Plural: If I had two daughters who were cute, I would be very happy.
Some native speakers, potentially Americans more than British, might say that 'was' would also be correct. However, the intention of your statement is presumably 100% hypothetical, insinuating that you are never going to have a cute daughter. The subjunctive should be used.
The most famous exemplary sentence of the subjunctive is probably 'If I were you I would...'
I believe that in modern colloquial English, even native speakers sometimes tend to replace 'were' with 'is' or 'was'.