Love Spells are quite tricky to perform as they are easily muddled up due to how complicated they actually are. There are many factors to take into account when thinking of casting a love spell and you might find that not many witches will perform them because there are so many things that could go wrong with them no matter how well you try to make sure you have covered all angles.
The main issue people have with love spells is that they go against Free Will (because let’s face it, if you have to cast a spell to make somebody love you then there is a problem even before you have done any spells.) However from what you have said about your issue I think it is clearly a case of a casting gone askew as not only has it made the recipient of your affections a little 'full on' with you, it has also made his little sister become a bit obsessed with you too. So this is not a good situation.
While it is clear you never intended to hurt anybody, casting a love spell, especially if the person you are casting it upon is already involved with somebody else, means that the other person is undoubtedly going to end up being hurt.
This is another reason not many witches like to use love spells, as although not all witches are Wiccan and many do not follow the law of free will it is pretty much a universal morality that causing harm isn’t a positive thing to do.
Unfortunately there isn't much I can suggest in regards to the other girl this boy was previously with, as it’s not like you can just rock up to her and explain that you cast a love spell on him which resulted in him leaving her! But in regards to things between you and him, I would personally suggest reversing the spell.
If you reverse the magick it should stop all of the 'full on' behaviour from him and then that way you can build your relationship up naturally. It will also help to straighten out the issue of his sister being obsessed with you too, as the effects on her are from the same spell. (Which does make me wonder - when you did the casting, did you specifically cast it on him only or did you use a generic love spell? I am inclined to think that it was the latter since his sister is also showing signs of being affected after handling your pendulum which you dipped into the drink that you created as the spell.)
Alongside the drink and the pendulum...were there any other workings that you used? Any words, herbs, crystals etc. that you could use again but to reverse it? If not, you could just do a general reversal spell (it doesn't have to be anything intricate or fancy as after all magick is pretty much all about your intent).
I use a simple reversal casting where I light a black candle inscribed with what I want to reverse. In your case I would suggest you inscribe the affliction and the names of those involved. Once lit you can ask the Universe (or whatever you choose to use) to help reverse the effects of the spell and to take away any affection created by the spell. Once the working is reversed then if you guys are destined to be together you will do so in your own time and pace and it will be true, not the result of any castings.
If you want to use magick to aid your relationship perhaps perform a promise casting, where you put it out into the Universe that you want to be together and the direction you both want the relationship to take. Try both lighting a white and a red candle (white to symbolise the universe, red for love) and then holding these in each hand while speaking out loud your feelings and wishes for each other and the relationship. Allow the candles to burn down and then either tie them together with red ribbon or cord and keep your own sets with you (or swap so he has yours and you have his), or bury them together in a place that is special to you both. If you don't feel comfortable saying those things out loud then you could write it down and burn it using the candles (don't forget to keep yourself and your surroundings safe if you burn anything...and don't forget about your fingers!)
As with any magick I always suggest that you go with your gut! Your intuition will lead you to what you know is the right thing to do. But if this doesn't sit with you right or make you feel happy then you should reverse it, not only for the other people involved but also for yourself too!
I hope this helps you, and I really hope you manage to figure out this situation with yourself and this boy (and not forgetting his little sister). If you ever need to ask anything else, then feel free to contact me again and I will be happy to help in any way I can.