1st off your sister didnt force your ex to have sex or almost have sex with him. she might have made her self available,

dont take his lies and drop him. your sis did you a favor cause if he would have sex with her then he would have no problem doing anyone else he could. you found out befor you are marryed but you have another problem you have a child by the man. now you have to decied if you want to keep it or not. this is totally up to you. i know if i was planing on marrying someone i sure as heck wouldnt be in bed with their sister for no reason in the world.

well sense you are wanting to be nice you must still care about him. you should not lower yourself and go out on him or anything of that sort. ask him out for a lunch or other dry meeting place and just come out and tell him that you do not think your lifes are going the same direction and you think that it would be best if you ended the relationship. dont argue or start the blame game that only dirtys the memories you have of the person. if you try and end it by email, phone or other inpersonal way it will not finalize the relationship. also be true to yourself. dont carry on the thing just because you know it will hurt him/you to end it.