Parents are not their kid's best friend. Whether they are a SParent or a BioParent.  Your SO has to put you and his relationship with you above all else.  The partners and their relationship are the top priority. Not children. Regardless of kid biology.

Kids are the top relationship responsibility but not the priority.

Many SParents go into the whole blended family adventure with the rose colored glasses of being their SKid's BFF and sadly, those SParents are often the ones who end up as a serial X for the entitlement focused prior failed family breeder.

Protect yourself and future. 

Good luck.

Not to sound negative but...welcome to hell

Unfortuntely you came into this at a bad time. That age range, 5 to 8 years old is a horrible time. The kid isn't cute anymore but not yet independent either. They consantly talk about the stupidest shit and constantly want validation on said stupid shit. They don't care about anyone else because they haven't developed the cognitive capacity to be aware of anyone else's needs which makes them selfish assholes. 

You can't force a bestie relationship into existence. And you will never love her like she's your own. Hell, you might not even like her once you get to know her. You also can't stop the jealous feelings you'll have. 5 year olds love to steal attention away and will do so on purpose. 

You also can never fully accept you'll permantely be on the back burner. You'll be the Mrs #2. The after thought. You'll either learn to deal with it or you'll get so resentful you'll leave. You have to realize you're putting YOUR LIFE on the back burner as well because all plans you had will either revolve around a kid who isn't yours or will be thwarted all together because of a kid who isn't yours. 

Give it a fair chance but don't enter into this thinking it will be sunshine and rainbows. It's unenjoyable for a lot of step parents, a lot of which wish they never tried it (including myself). If you find it's not for you, don't hesitate to leave. 

I would enter this very slowly and extremely cautiously!  You dont want to be the kids best friend, trust us , it will get you nowwhere.  You are new and fun and she most likely does not understand all that is going on at the moment.  When she realises that you are her dads new girlfriend, thats when the games begin.    I would take a step back and watch how your boyfriend parents her and that he enforces rules .  good luck  

You sound like you have a good head on your shoulders and are smart to ask for advice.

My first bit of advice will be about your relationship. Kids don't come first (I say that as a mother). Our children are our first responsibility, but our marriage has to be our first priority. 

We take care of our kids needs above our own, but we have to prioritize our marriages needs above our kids wants. I hope that makes sense! 

2nd- I would suggest keeping in the "fun aunt" relationship with your future SD. You only have her every other weekend and split holiday's. Let your SO be the discipline and hold him accountable to it- privately. Just try and make the time with her fun and enjoyable for you both. 

You sound a lovely person and all I can add is come to this site with any problems you encounter along the way. A lot of us here are very old hands at the step parenting lark!   Good luck with it all and try not to encourage your partner to make his daughter the centre of the Universe.  This won't help anybody.  

My best piece of advice to you right now is to take and keep off your rose-colored glasses.

You two have been together for a year. You're about to hit that transition time between honeymoon phase and long-term relationship phase. This is the time when you need to start paying active attention to what your BF does versus what he says. This is when you should start staying with him for longer periods of time when he has his daughter (like over the holidays) to see how he parents long-term. 

Of course you've bonded to his daughter. You have seen her on a weekend when you and her dad aren't busy and can plan fun stuff. You're also new, and kids tend to be nicer to new people. They also tend to want attention from new people because they're new.

Right now, you should be doing a lot of observing. Watch how he interacts with his daughter. Listen to how he talks to and about her. Ask him questions about why he says/does things. Talk to him about what he sees your role is in helping raise her. Take note of how he talks to and interacts with BM.

You've established that you and he are compatible without the added stress of a younger child and all the drama that comes with stepkids. Take the next year to do the same research you (hopefully) did over the last year vetting him as a good partner. Now you're vetting him as a good partner and parent. You can't divorce those personas from one another, and being bad as a parent will bleed over into being a bad partner.

Don't be the best friend. Don't let him set you entirely aside for his child and her wants. It is NOT your job to make his life as a single parent easier. It is HIS JOB to figure out how he can be 100% parent and 100% partner. That is HIS cross to bear, so don't overstep and take too much away from him. It not only hurts your relationship long-term, but hurts his relationship with his daughter, too.

My advice would be to read posts and learn about potential issues from that reading. Many things that you would never consider an issue crop up like poisonous mushrooms.

Some additional considerations:

- Finances. Things tend to get tricky with a person who is a parent. Here in the Us there is child support, health insurance, life insurance and all that. We keep our finances separate and pay 50/50. Things to watch out for are when a bio parent is every other weekend and holidays - they start to become a "disney parent" or "atm parent" only good for the good times and the $$$. And thats where you need to watch the financial part. If shes sweett now, you will be like me and spend your money on her. And then if the bio mom is toxic she wont like that!!!! So lots to consider there.

- Marriage and babies. This will take a lot of thought and consideration and comunication. Blended families are the toughest model to successfully plow through.

SO - Good luck and post questions and read read read.

Welcome to the site, Mona! I admire your sensible approach to your relationship; it shows maturity and foresight. However, there is an area of concern with your statement:

I would be happy to be the one who's gonna end up to pay more for everything just because of the above reasons.

 Mona, you have no financial responsibility for someone else’s child. The girl has two involved parents to cover her expenses. Living costs should be shared between you and your boyfriend but NOT expenses for your boyfriend’s child. 

What begins as generosity on the part of a stepparent can often become an expectation on the part of a biological parent.