Oh OP. 
The most important thing to take away from this is that your ex is a grade A cunt.
Next up: please make contact with local MH agencies, or Womens Ad, or just book a double appointment with your (preferably female) GP. Any of these contacts can signpost you to an EXPERIENCED therapist, who can support you with regard to the historic abuse.
It's very important that you get a recommended therapist, who is well-versed in the impact of CSA * & working with adult survivors. Hence going through channels to find a well-recommended one.
Once you have that therapist in place, & feel comfortable with them (btw - it is totally acceptable to meet more than 1, until you find one you 'gel' with), you can explore your desire to tell DP about what happened to you as a child.
Do you feel able to do that, or is the desire to tell him burning you up? If it is - PLEASE be wary of it. There is nothing wrong with telling him, but the desire to do so may be driven by complicated factors. These could include the irrational (but normal, & understandable!) wish to compensate for your ex's appalling reaction, or the wish (again normal, & understandable!) to be 'forgiven' for any irrational feelings of complicity in the abuse.
I want to be really clear & direct here - YOU ARE NOT RESPONSIBLE OR IN ANY WAY AT FAULT for what your brother perpetrated on you. Please let that thought trickle down into the core of you.
It's even possible that your brother, being only a youngster himself, was also being sexually molested by somebody, hence going on to act out with you. But THAT is not your problem, do not get caught up with that, it's time to take care of yourself, & yourself only.
However - it's a huge thing for your DP to deal with, if & when you are ready to tell him. He will have his own complicated feelings about it, & it will take him time to process those feelings. So YOU need to be professionally supported, should you choose do tell him.
Please remember - this is YOUR life.
You don't owe anyone, even the most loving & supportive anyone, this part of your life story.
You do not need 'forgiveness', you are not dirty, the shame belongs 100% to your brother.
It is not up to other people to have an opinion on what happened to you, & you do not need their feedback.
They will have opinions, & possibly even feedback - it's a hard thing to hear about somebody you care about - but what YOU feel, & how YOU manage your feelings, is of paramount importance.
*CSA = Childhood Sexual Abuse, for those unfamiliar.
OP - I am a fellow survivor. I don't know where I'd be without therapy. Take it from me - the kind of professionals who counsel survivors are amazing, You WILL be believed, supported, encouraged. You WILL be dealt with with compassion. Please take those first steps via your GP, or other agency, this week.
napac.org.uk/