[pirinosa]

Let users who have experience with this kind of situation give suggestions to the brother, my humble request.

Start with by asking help from Allah SWT.
1. Prepare yourself to be humbled, spoken ill of and even be cussed at by your sis.
2. The only thing going for you is you are elder to her.
3. If you are going to be even slightly angry towards her, believe me you are going to lose of whatever thats remaining.
4. The threat of suicide which your mom hints at is NO JOKE, its is very real and she is not even blackmailing you. Remove that thought from your head.
5. Ask for forgiveness from Allah SWT first and then your sis, I'm not kidding, ask for forgiveness with all humility, she may not even give a damn at the beginning, don't care, ask again and again.
6. What she might very probably do is gonna have a go at you, recalling your past, have patience, she'll be out of steam (anger) after a while.
7. (As sister ARAM said) Sit with her, ask her what the problem all about, gain her trust, her confidence.
8. Don't YOU ever talk about Deen to her at this juncture. You are not the right guy, sorry to say that bro, its written all over your very honest post.
9. Get a 3rd person involved here, preferably an elderly khaandaani male relative, somebody she respects and fears, not the gossip mongers, definitely not, remember that she is a girl, its about her honor and when word about all this gets out as gossip, she won't be able to show her face again to anyone, even if she comes out of it successfully.
Act as if everything is as normal as it can be.
10. Tell your younger sis to be totally be out of this and request her to do full on khidmah of her elder sis, do all this with full on brotherly love.
11. Don't try getting the details of the boy, yet, 'coz I don't think with your history she'll be comfortable giving it.
12. This situation can go out of hand if not handled with loads of patience, if handled well she is going to thank you all her life, if not curse you all her life.
13. Have faith in Allah SWT, and make dua, give sadqah, give it regularly, and janab, Allah SWT has kept our success in following his Deen, and every path followed other than that may only give you temporary success or may not even give you even that, how is it we call ourselves muslims and fail to follow His words and then if any calamity befalls us, we complain 'This too is from Allah SWT'??! Isn't that absurd?.