Upholding the ties of kinship is obligatory to the extent that one is able to do it
Question

I have sisters who are married and my mother is married to someone other than my father, because my father is deceased. I work in the army and I want to go and see them but my circumstances do not permit it. I am also married and if I go there and leave my family, I would have to stay there for at least three days, during which time I would be worrying about my wife and children. Am I severing the ties of kinship, given that I have not been in touch with them for ten months?.
Praise be to Allah.Upholding the ties of kinship is obligatory to the extent that one is able to do it, starting with the closest and then the next closest. There is a great deal of goodness in it and it serves many interests. Severing the ties of kinship is a major sin, because Allah says (interpretation of the meaning): 
“Would you then, if you were given the authority, do mischief in the land, and sever your ties of kinship?
23. Such are they whom Allah has cursed, so that He has made them deaf and blinded their sight”
[Muhammad 47:22]
And the Prophet (peace and blessings of Allah be upon him) said: “No one who severs the ties of kinship will enter Paradise.” Narrated by Muslim in his Saheeh. 
When a man asked him, “O Messenger of Allah, whom should I honour?” He said: “Your mother.” He said: “Then whom?” He said: “Your mother.” He said: “Then whom?” He said: “Your mother.” He said: “Then whom?” The fourth time he said: “Your father, then the next closest and the next closest.” Also narrated by Muslim. 
In al-Saheeh it is narrated that the Prophet (peace and blessings of Allah be upon him) said: “Whoever would like his provision to be increased and his lifespan to be extended, let him uphold the ties of kinship.” 
There are many ahaadeeth on this topic. What you have to do is to uphold the ties of kinship to the extent that you are able to, by visiting them if possible, or by writing or phoning. It is also prescribed for you to uphold kinship ties financially too, if the relative is poor. Allah says (interpretation of the meaning): 
“So keep your duty to Allah and fear Him as much as you can”
[al-Taghaabun 64:16]
“Allah burdens not a person beyond his scope”
[al-Baqarah 2:286]
And the Prophet (peace and blessings of Allah be upon him) said: “When I command you to do something, do as much of it as you can.” Saheeh – agreed upon. 
May Allah help us all to do that which pleases Him.