This is a more complicated query than it may at first appear- daughters and abstinence.
It covers topics from religion and faith, sexuality and maturity, to pressure or guidance as well as questioning when the suitable time is to approach each of these as well as other areas.
I can only draw on what you've said and read into it as it's not outlined too clearly- but the point being, Abstinence itself has strong ties to faith and religion- it's possible to want Abstinence without being inherently religious, but the question is: is your family religious?
If you practice at all, either at home or by visiting a place of worship, I'm sure that they'll bring up the topic themselves at some point, or you could possibly inquire to see if there's anything that will be approached in the future.
As more clarity is needed in this field, I'll leave it at that.
Sexuality and Maturity
At their existing age, it's safe to say that the most they've heard of is boys kissing girls and that's hopefully it- their school will approach the topic themselves sooner than later and that's when it would be best to also approach things at home as well. They'll have questions, you can give answers. You may be able to find some literature to work through from the school, or through other avenues such as the almighty internet as well as potentially your Doctor if needs be.
It can give you an insight into some topics to approach and potentially how, without trying to enter the field at the age they are currently as this would either not be taken on board, or cause more trouble than actual help as it can be confusing and too much.
Bear in mind that there's every chance they'd be more comfortable speaking to their Mother about it, rather than their Father- this is very much an Apples and Oranges and case-by-case basis thing as this could be the other way around- but if they feel comfortable about it with you, there's no harm in that. Just be supportive and understanding and give them a chance to come to you in their own time and way rather than push.
They aren't at an age now where it's probably appropriate to worry or have any approaches yet, perhaps ask at their school when the topic is going to become involved and work your way from there. As they grow older and will understand more and in better detail what's being discussed, it could be that you won't need to expand on much at all, they'll draw more from your behavior and example as a familiar foundation than directly ask.
Pressure and Guidance
The whole process of someone learning the "ways of the world" as I'll put it- isn't always one of easy flows and a dynamic and wholesome experience. Some teachers, religions and families deal with the topic in drastically different ways. Some leave it entirely down to the schools, others down to their religious choices and some won't even approach the topic and just leave the kids to work it out themselves.
However, letting them know that you're there and you're approachable about it is what's important. You can't always help if one or both are shy and feel embarrassed or ashamed to ask things, but I'm sure your relationship is strong enough that if they want to, they will.
Throughout their development there's no telling precisely how they'll mature and behave- it could be that they'll rebel and throw the "shackles" as they could see it of religious and parental restrictions and push the boundaries, or they could move in the same steps as you and be sensible, there's no way of knowing.
What is important is giving them the chance to make their own, informed decisions, with the correct and relevant information available. Be fair and non-judgemental, it isn't just Boys that are the one's that push sometimes, there is such a thing as teasing (and the resultant issues that can cause!), if you do go that far here's how you can be safe, "no" is always an option, there's no rush to do anything, etc.
Ultimately a lot of it comes down to choice- they always have a choice, but you're their parents and are there for support, guidance, comfort and help. They can choose to do what they feel is right or what they want, but you're there to give advice if it's needed.