what to do

I have been with my SO for about 5 years he has 2 kids to an previous relationship. His kids are with us every other week and I love having them around. He is working 2nd shift so I provide most of there care but I can't help but feel like I want one of my own. financially it would not be responsible of us and we both know it. also his kids are getting older and I don't want them to have some crazy age gap. What would you do? **the reason for the concern of the age gap is both my SO and I are 1 of 6 children ourselves. his oldest brother never excepted him because of the 19 year difference. and to even go into my siblings would plow minds let's just say I didn't get to know my older sister till we were adults and my youngest 2 brothers I never really got close with because what is there to talk about when there's this much gap.