To try or not to try....

Hey ladies
For those of you that dont know me i'm Eve and im 19 i have gorgeous ID twin girls who are almost 14month. I've been with my partner sam since the girls were 4 weeks old but he is not their biological dad but he does a good job at raising them. Im not sure what to do whether to try for another baby or not yet. I want to have Sam's baby and he wants one too but its a case of timing i'll explain below...

Thre girls are growing and i love them but they are hard work. Im looking after them and im doing a foundation degree at college and Sam is at uni 3 days a week. He has another year to do on his degree. I want to go to uni too and do my nursing degree. However Sam really wants a baby, as the twins arent his i think he longs to have a baby of his own type thing. Though he is the twins dad as their biological father has little to nothing to do with them. Sam's been there since the girls were 4 weeks old so he knows how hard it is and things. He's not naive in it, I do want another baby i always have but its a question of when the right time is! Financially atm we survive and thats about it we have the occasional treat i mean better now im back at work part time and that but still. I dont know if i could afford another one and what if i fell with twins again? One fine but another two right now id die i think. But then sam was saying do you want them to be so far apart or do you want them to play together because theres a limit of age difference before they wont get on. Sam also said we could start trying and it might take a while for it to happen then i said or it could happen tomorrow after all i wasnt trying with the twins i was actively trying to prevent it and it still happened. So im in a bit of a pickle surely if i was ready now i'd know i wouldnt be thinking about it would i? Or is it natural? I mean i miss the girls being tiny but i like my sleep and i dont miss the labour and the 9month previous, i dont miss the sleepless nights and the endless crying. I dont miss fighting to get them into a routine but i do miss just watching them sleep, i miss walking around the shops when they were asleep knowing they couldnt grab anything, i miss them being dependant on me for everything. So im balancing up. I just dont know what do you girls think...I mean im 19 i have plenty of time to have more my biological clock isnt pushed if you know what i mean if i hadnt had the twins till i was 38 and i wanted more then id be thinking right do it now but im not sure. I do want more kids thats not what im saying and i would love to have one with Sam, but we cant agree on timing, I am also slightly concerned that if i have Sam's baby he will favouritise that baby over the twins because that baby is biologically his. I dont think he would but i see his point he has no rights to the twins at all! He cant consent to medical treatment he cant sign forms for nursery he has to provide a password to pick them up from nursery because he isnt their legal guardian or parent and he feels a bit out of it sometimes. But the girls think of him as their daddy no doubt and i call him daddy in front of them IE go see daddy or give daddy a cuddle since their real dad is not around i see no harm in it. Their real dad doesnt want anything to do with them see. Hes too busy with his son. Ive seen it happen he was great until his new gf fell pregnant and had a boy then the girls were second best and i dont want that to happen to them again. Am i being stupid?
Part of me just thinks that i should ask Sam to adopt the twins then he is legally their gurdian im not sure how it works like but i could find out and then he isnt out of it and we dont have to have a baby for him to feel responsible as well as me. I dont know
Opinions welcome ladies
Sorry for the long post i just dont know what to do....
xEve, Ella & Ruby x