Languages don't actually have ages, and they aren't discrete things like people are. So we'll have to first interpret what you mean by "older language" versus "younger language". It's even harder to understand what the difference would be between X being an ancestor of Y versus X being an older variety of Y.
Let's take "American English as currently spoken in the Pacific Northwest" ("Y") to be the younger language, and "English spoken in the British Isles in 1600" ("X") to be the older language. It would be reasonable to say that X is an ancestor of Y, in the same way that one can say that Knudt Knudtsen is an ancestor of me. We would not say that I am a newer variety of Knudt Knudtsen, because Knudt Knudtsen is a discrete entity who clearly died. X is not a discrete entity which clearly died at some point in the past. What an academic linguist would conclude is that there isn't a difference between the possibilities that you describe -- they refer to the same fact.
The same problem afflicts appeal to linguistic "sibling", which would probably be taken to be "a closely related language descended from a single earlier language". Well, it turns out that "American English as currently spoken in the Pacific Northwest" ("Y") isn't entirely uniform – there are documented dialect differences (east of Cascades / west of Cascades being prominent; there is an Enumclaw dialect). No two people speak the exact same language (i.e. have the exact same grammar and lexicon, even setting aside cases where one person knows a word that the other person doesn't know). The significance of that is that there is no clearly-defined boundary between language and dialect. There seems to be a feeling that Scots is not a dialect of English, and certainly I can't understand it at least the bit of it that I've heard. So by the mutual intelligibility test, most people would say that Scots is a distinct language from English. I don't think there is much support for thinking that [Geordie][1] is held to be a separate language, but it is completely unintelligible to me. Indeed, a number UK dialects are unintelligible to me, and some US dialects are too.
A problem with using popular opinion as the adjudicator of the dialect / language distinction is that people mostly don't have opinions except about their own language, and then their opinions are shaped by completely unscientific and non-linguistic criteria. Whether or not the various Kurdish languages and Saami languages are considered to be separate languages or dialects depends on non-linguistic ideology and political matters. (My grasp of the situation is that Kurds downplay dialect differences in favor of a single Kurdish ethnicity, because there is an ongoing struggle for independence that relies on the unity of the Kurds as a nation; whereas the Saami enjoy basic rights and a fair degree of political autonomy so recognition of linguistic differences is not a threat to their future. More could be said, but won't be).
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I don't think using "referred to as X by its speakers and outsiders" will get anywhere, since people's name for their own language is frequently different from what outsiders call them (the general scheme is that the language-internal name translates as something like "language of the people" and the outsider name translates as something like "language of the barbarians"). Eskimo, Sioux, Berber, Hottentot and Lappish are all etymologically insulting exonyms, likewise the Russian word for "German".
If you discard outsider names and stick with self-names like Suomi (Finnish), Deutsch (German), ελληνικά (Greek), or Italian(o), then there might be a legitimate question of cultural identity. Perhaps we could say that "English" first existed when those Germanic tribes called themselves "English" rather than "Saxon" or "Jute" (or historical equivalents -- "English" is a modern word). Note that Irish (Gaeilge) and Scots Gaelic (Gàidhlig) are different languages, but the names of the languages reflect a continuous use of the indigenous name for the language. A historical change in the name of a language probably reflects a major cultural change more than anything else, although actual change in language can be a part of cultural change.
Anyhow, the way "ancestor" language is used in linguistics, it refers to what preceded historically, without care for the language / dialect division, though we tend to reserve the word for predecessors that are at least some number of hundreds of years old.
