Hi, Nokaze. "If I was your mom..."-is the 2nd Conditional, you are right that "were" in such setnences is used more frequestly, though, "was" is also correct as an option --But you're not, though.-"though" here is used for emphasis with no specific meaning of its own just to highlight the disagreement expressed with "but" (can only be used like this in a colloquial speech) Though can also be used after adding a fact, opinion, or question which seems surprising after what you have just said, or which makes what you have just said seem less true. Examples: I think she's Swiss. I'm not sure though. Two heart attacks in a year. It hasn’t stopped him smoking, though. Hope that'll help))

see here: https://webapps.towson.edu/ows/were_where_we%27re.htm For a simple introduction. Was is past for = if I actually were (are in the past) your mum. were can be used also in the sentence. I think was is being used more often than it used to be, among younger speakers. especially on the Television and films etc. a slightly more detailed introduction: https://www.grammarly.com/blog/was-vs-were/

If I was (singular past tense of the verb to be) your mum. But you're (singular) not though. If we were (plural past tense of the verb to be) your parents. But you're (plural) not though. It is a quirk of the English language that the second person singular verb TO BE is expressed as You're. For the second sentence, though is used for emphasis and is optional. The sentence "but you're not" is also correct.