Step parenting is rarely ever ideal, rarely stable nor the mix - getting all things right.
I've been a step parent twice and I've had a partner to my kids- neither worked out in terms of slotting into a united family. So I'll be brutally honest with my feelings.
It is normal to feel jealousy towards the children of your partner after all those kids, as lovely as they are, take time away from your both.
You are only 21yo, there is no need to rush into even thinking about children at this time. With the potential powderkeg in the future with you both not getting the chemistry right you should get things ok now and for a few years before you go down that path.
A step parent will always be a step parent unless- the other birth parent passes on or the child severs ties permanently. My daughter severed ties with her birth mum, she now calls my 2nd wife "mum", for example. Unless one of those things happen you'll have to accept that your role is step parent and that means not being fully in the loop in regards to the childrens needs, putting it more bluntly- you'll be locked out of many conversations legitimately. Many times you might be told by your partner or the other birth parent to "butt out" of decisions even though they'll effect you. EG You have planned a night out with your partner, your anniversary, then 2 hours earlier the other parnet rings your partner and he/she want to attend a parent and teacher night that evening. Well your partner will be the meat in the sandwich and might feel that is more important. You will be disappointed but you must be flexible as a step parent and decide to make it another night. Time and time again my ex partners could not or would not be flexible and keep to their role as a step parent.
My other advice is to actually embrace that role. Read to the children, be their best friend, essentially aim for your role to be so good that those kids will relish in the thought that they will visit you even more eager than seeing their father.
Summary- be flexible with the visitation routine, draw a line in terms of being too involved in your partners communication and decisions with the kids and his ex, be a playful friendly step mum. As your relationship is more likely to fail than a non step children couple- be wary that you both need a lot more time to see if it all works out before having children of your own. Seek a family counselor to fine tune some problems with your partners lack of regard to you.