HELP
Ok, I'm 22 been with SO for 5 years, (he's 23) we are 2 kids deep. MIL told me 4 years ago on a drunk night she had another child before him, a daughter, she was raped, gave birth to the baby, gave it away to the person who raped her..minutes after giving birth. SO knows she was raped but doesn't know about his mother having another child before him and handing it over, also she was paying money monthly, and the rapist dad was sending photos in return, the daughter is 27 now I think, SO has no clue..neither does the daughter..MIL brings up this daughter everytime she has a drink when SO is not there of course..says she regrets it which I understand. But a lot of the time SO will bring up the fact that he was a only child and wishes he had siblings (MIL brushes it off), we will also sit together and watch long lost family (awkward) and she will say I'm the daughter she "never had". I've tried to get her to tell him but she doesn't want to and says even when she dies she doesn't want him to know. I wish she never told me and now I feel bad and tempted a lot of the time to tell him but I won't because I know it will cause an erupt situation. When she told me originally she made me swear to never tell him but I feel so bad keeping this from him and if it was me and my mum had told him that I know he'd tell me!...well I hope. Just needed to get this off my chest because it haunts me every day. What would you guys do?!!!! Advice please.