I don't think there're any problems with the situations you've described. I actually have two very similar scenarios in my own game (your best friend has a crush on one of your potential love interests, and your male childhood friend will support your relationships with other guys), so I'd definitely say go for it!

Would you and your sister be in love with the same guy at the same time, or just in different paths? The former could be an interesting setup that leads to lots of love triangle drama. Personally, though, I would not want to be on a path where I would have to hurt my friend to get a guy. Like I was always sad in Tokimeki Memorial when one of my girl friends would enter "Rival Mode" and we'd stop being friends because we liked the same guy - I often thought I'd rather help her get with him instead of fighting. ;_;
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Why not give the player the choice? Have little sister confide her feelings for childhood friend... and give a supportive option and a 'wtf?' option. And maybe the 'wtf' option could derail the MC back onto childhood friend's path, if it turns out that they're jealous of little sister's crush...
I second this! At least in my game, I'm letting the player choose what they want to do in the end – either you support your friend, or pursue the relationship with the guy. And if you do want to pursue the relationship, you'll have to work things out with your friend first, so yeah.
As for some love interests appearing in other paths, that's awesome! I wouldn't want one of my good friends to disappear from the game simply because I didn't go down his route (and frankly, I’d like him better if he did support me no matter what ^_^). As long as the guy friend’s actions stay consistent with his personality and relationship to the MC, it should be fine. Like your nice sweet friend shouldn't suddenly morph into a crazy jealous villain just to provide conflict in another guy's path, unless there is a very good explanation behind it (though I guess the yandere type appeals to some people haha). I also think the friend's subplot shouldn't dominate the other guy's path unless they're somehow interrelated, else it's not really fair to that other guy. ^^;
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Or destroy the good relationship with the sister AND makinf the friendship go awkward since the MC actively meddled.
Oh no, that sounds so terrible! I'm sorry. ;_; I actually hadn’t thought of that option before, but it seems quite realistic. Hmm…I just may have to use that at some point… ^^