As far as determining the mother, the above post is correct. My mice colonies nurse communally, meaning that all of the females nurse all of the babies, not just their own. If your mice are mature, which it sounds like they very well could be, then there is a good chance that it will not matter which mice you take from the cage because the others will pick up the slack.
I typically remove my weaned mice from the breeding colony between 3 and 4 weeks old. Most of the mice sites that I have looked at suggest waiting until 4 weeks, but I have had no ill effects from separating as early as 3 weeks.
Hope this helps,
John
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