Dating boundaries

[The Blade]

I don't mean personality-wise. Almost every person, except for the extremely shallow type, has some (kind of) people he/she wouldn't date (I mean date to a, more or less, serious degree; not just have sex with for a night or few times at most).
Here's my idea:
If you are a man, would you date or have you dated a chick who is among your ex's closest friends? Equivalental question to ladies here.
Males here, would you date the ex of a close friend of yours (obviously guys you know but say just a few words to when you accidentally meet them don't count as such)? Equivalental question to women on TA.
Men, would you go out with your ex's sister? Equivalental question to women here.
My experience and opinion on these:
1) I once dated a very close friend of a chick I had recently broken up with. I don't regret it and personality-wise she was more of my type than my ex. Later, when we were no longer together, I gathered with my ex, thinking she might had changed some things in her personality (as she said) which led to our previous breakup. She hadn't, so we separated again. After this I decided that returning to an ex isn't worth it. These two women, of course, stopped being friends but that's how it goes.
2) I can't imagine dating the ex of a friend of mine freshly after they have separated, regardless of how long the relationship lasted. I haven't faced that dilemma but I only see this as an option if more than a year (closer to two) has passed since their breakup, none of them has feelings anymore and I know the breakup wasn't the chick's fault. Now, I've had a woman who had gone out several times with a good friend of mine asking me to fuck her but, of course, I told her it wasn't gonna happen. Not only I oppose betraying a friend like that (I told him about it the next day and he stopped dealing with her) but I was myself going out with another woman at the moment. My philosophy is that if you don't wanna be with someone anymore, end the relationship first and then deal with other women (if you want).
3) The question about your ex's sister I made part of this topic due to a conversation I had some time ago with another friend of mine. I had mentioned the guy that I had had some (not really serious) chats with the sister of another ex of mine (she had initiated contact) and he came up with (I know him; he wasn't joking) ''That could be your next girlfriend.''. I was like ''Hell no, man, definitely don't wanna mess in these things.''. I still find it inappropriate. Wonder if there's anyone on this forum who'd do this.

Share your thoughts.

[Benyzero]

Once I fucked a chick who was a girlfriend of my once best friends girlfriend. The chick was literally jealous of her, and asked me awkward questions like who is more better looking, etc. Damn she was into me big time

[Alexandro]

1. Fuck no, my ex girlfriend was Portuguese so they all gossip with each other, haha.

2. This really depends on the circumstance and how cordial their breakup was, but in most cases, probably not.

3. No, that would be quite awkward for both parties considering my relationship with her sister (my ex) did not end amicably and they are rather close to each other.

[Hithaeglir]

A friend's partner will register as a new friend and this will never change even if a separation occurs in the future. It's not a case of boundaries but rather how my subconscious works.