I don't think it would be weird, but I would evaluate how much I was able to express antenatally compared to how much I was thinking of giving #1, and how much baby #2 would benefit from it and how much #1 would benefit, make my decision based on those figures.

I was only able to express 1-2ml per day, and so I only had about 30mls of colostrum in my freezer stock by the time DS came along. i don't think I would have been prepared to give any of that to someone else, though I don't have an older child, so I can't really place myself in your situation. If I had been expressing 5ml a day, or 10 ml a day, or something, then I would have been more willing to use some for purposes other than the new baby on it;s way.