義妹 / 義理の妹/姉 question

I was looking up the meanings of 義妹 and 義理の妹/姉.

About 義妹 - I understand that it could mean "sister-in-law" (義理の妹。夫または妻の妹、弟の妻など), however, I'm more interested in the 2nd meaning. 「血縁関係はないが、姉妹の約束を交わして妹としてい る人。妹分」or 「他人同士でありながら、姉妹の約束を結んで妹となっ た人。妹分。」
Can someone tell me what「姉妹の約束を交わして」 or 「姉妹の約束を結んで妹と」exactly mean?. The 妹分 in the end kind of confused me. Not sure if some person becomes an actual sister (by law) or " someone who one considers a sister (妹分)"

About 義理の妹/姉, I can't really find any reliable dictionary entry about this, but apparently it can mean "one's sister-in-law
". While jisho has the "step-sister" meaning listed as well - Find words - Denshi Jisho. If anyone could clarify the meaning of it, I would appreciate it.

妹分 is there in the dictionary to indicate that this meaning of 義妹 is similar to 妹分 - it's not part of the sentence that preceeds it. It's not talking about any sort of legal arrangement - more the "close as sisters" type of situation.

義理の(relative) generally indicates a relative by law (normally marriage) rather than by blood. Fun fact: "in-law" used to be used the same way in English, so "son-in-law" could be used where we'd say "step-son".

Ok so, let me get this straight. 義妹 = "姉妹の約束を交わして妹としてい る人" means that someone just agrees with someone else to become "close as a sister" and can't mean a adopted/legal/step sister?
Asking because I don't know if someone is considered to be a real/adopted sister if 義妹 is read as いもうと instead of ぎまい.

While 義理の～ can mean adopted/step-sister, brother, etc? I had no idea that "in-law" could mean "step-～" in English, haha.

There are two meanings listed for 義妹, right? The legal connotations are covered under the first definition of 義妹 (basically 義理の妹). In this sense 義妹 and 義理の妹 seem to me to be pretty much the same - someone to whom you are not related by blood, but through other family connections (typically marriage). It can be the "sister-in-law", "step-sister", or through adoption. In this situation it is possible for 義妹 to be read いもうと (and for the other similar 義○ compounds to be read using ○ reading alone).

Great, thanks for clearing it up.