SIngle Moms with 3 or more children and going to College

I am a widow who has 3 girls ages 11,10 and 3. My 2 oldest girls daddy,which was my husband was killed in a car accident 9/2004. I have recently had another baby girl who will be 2 August 1st and the father isn't much help with her or any of us. I have made it this far after my husband passed away at the age of 26 but now I have another little one,I just cant seem to get everything back together. My 2 older girls love their baby sister more than anything. That's a plus but they argue about everything with each other even though they are old enough to not be acting this way. I am currently going to school part time and trying to what I can. I just needed advice on how does everybody do it? I also have no family close enough to help me out with the girls. The older girls understand now what happened to their father because at the time of his death they were only 4&5yrs old,they don;t understand whats going on with the baby's daddy. I have tried to explain what I can and do what I can for all of 3 girls. i just need help with how to raise them by myself without making it so hard. Any ideas would be great.?i would like to know how to get the 2 older ones to quit fussing with each other about everything. I know they are sisters and are going to fuss but they do to much but yet they do everything for the baby and never fuss about her.They are such a big help with her and very good to baby. Thanks for the help.