Why welche and diese instead of sie?
Wir konnten keine Kinder bekommen, deswegen waren wir glücklich, als wir welche adoptieren konnten.
Meine Enkelin sieht genau wie ihre Mutter aus, als diese ein Baby war.
We couldn't have ( own) children, because of that we were happy then we had the change to adopt some (childeren).
in the first sentence welche takes the funktion of some. if you exchange welche with sie it means .....then we had the change to adopt them.
but als wir sie adoptieren konnten. sounds a little odd. the question popping up in my mind is who exactly is "sie"? we would only choose this variation in a conversation, if we were talking about the kids before. and the other person knows who exactly i am talking about.
in the second sentence it is only because diese sounds a little more distinguished then sie in this case.
You need "welche" in the first sentence because you are talking of taking in "any" children. "We weren't able to get children of our own ... when we could adopt some." These kids haven't been mentioned in this sentence before.
For the second sentence "diese" is a demonstrative pronoun which relates to the mother. If you used "sie" it would be unclear whether you were talking about "meine Enkelin" or "ihre Mutter" since both people are mentioned before.
jain... i think in this example (2.sentence) the mother is the only logical choise. so i think it would be fine. if i use sie in a sentence like this i always reffer to the person who was mentioned last. if i mean the person who i mentioned first i will mane that person again to avoid misunderstandings. plus: with diese you could also reffer to either of the 2 mentioned people (as long as they are both females). it isn't any clearer than sie. you can use diese/dieser or er/sie to clearify if you had one male and one female person mentioned. again; this is theory. in praxis most people i know will mention the person they want reffer to at last.