a crush on my ex's ex...help please?

I think how you act here may depend on how some of these relationships ended. I think at 16, it is not likely "love", but rather puppy love and infatuation that is more than likely to die down at some point. I think that may be important to weigh since you seem to already know what this relationship may cost you.

However, once you break up with someone, you don't owe them anything and especially they don't have control who you date or when. You should do what you want and not do what your ex thinks. The fact the she is suicidal suggests she has some problems that need to be addressed, and if you dating someone else is going to make her feel that way, you need to understand that it truly isn't your responsibility. She should seek out some counselling or therapy to help her manage her feelings.

So to summarize. I think you should go for it if its what you really want, but I also know how friends are at 16 and if you know the consequences of pursuing this relationship, you should factor that in because its probably not worth ruining all your relationships.