From time to time, I think about my mother who is no longer living.
I’m guessing that it’s “welche” here and not “sie” because the sentence is not making a distinction between a mother who’s still living and one that’s not. In other words, it’s functioning descriptively and not to point out or distinguish. Oder?
“Sie” would not work. You need a relative pronoun here, and you have the choice between “welche” and “die”.
With both pronouns it can be interpreted both ways, I guess just like the english sentence, either as an elaboration (“who btw. is dead”) or as a disambiguation (“the one (of my many mothers) who is dead”).
Of course, the disambiguation meaning is not likely - who has more than one mother?
As for “welche” vs. “die” - “welche” sounds more formal.
All in all putting it in a relative clause like that sounds a bit stilted, and I would just say “an meine tote/gestorbene/verstorbene Mutter”.
What is she like?
Is the English translation here supposed to be understood sarcastically? I.e. "why is she like this?, wie is sie so?.
(The fact that comments for different sentences end up in the same thread seems to be a bug in the forum?)
That’s a tough one to explain. This is not sarcastic at all. “Wie ist sie so?” is the correct and idiomatic way to ask “What is she like?”, if you don’t want to get too formal, for example by asking “Was für ein Mensch ist sie?”
“Why is she like this?” would be “Warum ist sie so?” (and the “so” would be a different meaning of “so”)
The “so” is not to be understood literally. “So” has unbelievably many usages, and in this case it’s a modifier that gives the question a vague character, and the question would sound odd without it.
If I’m not mistaken, it should be meaning I. 6b on this page:
For “Warum ist sie so?”, it’s literally “like this”. That should be meaning I. 2.
On Wiktionary (so - Wiktionary, the free dictionary) the closest is either Adverb #5, or the Particle definition. I’m not even sure.
Wiktionary calls it “expletive”, and I just learned that it’s a linguistic term meaning “filler word”, which has nothing to do with profanity 