Dear Dilip,
The problem that you have mentioned, though I share your feelings, being in a sort of dilemma of choice, whom to take along and whom to discard, yet I would prefer to call it a family problem requiring strict handling, of the situation, because it would not be solved astrologically. Astrology can only hint at the various problems that you may come across during a particular period and warns you in advance to take preventive action, if any, available for the native; and its (the forthcoming bad phase) linkages, if any, with the decisions / actions in the past. It cannot provide any effective remedy for any negative role assumed, which has brought poisonous consequences, of one's own making.
The problem that you are facing is a story of 20 % people who happens to be the only or main breadearner in the family containing aged parents, children, dependent siblings etc. I have received a large number of such cases where such natives are married to a girl from higher status in exchange of a fat dowry / other benefits. This bride once into the family would soon happen to take over the command of the family in her own hand with a little bit of pampering by the husband, and would demand everyone to follow her toes or get lost. She would soon raise her voice for coming out of the family responsibilities and leave the elders and dependent sibling mending for themselves. Everyone knows what will be the consequences if and when her demands are met. Towards her nefarious objectives she would not even spare her husband threatening to bring her influential relatives into the picture and take legal and other harmful actions. In these circumstances the womenfolk in the family gets mentally tortured for apparently no fault. And the husband ! He finds himself at the crossroads, like you. Neither he can leave his parents (who sacrificed their lives and comforts to bring up this child), nor the dependent siblings (for whom he feels his call of duties), nor can he manage effectively the erring and domineering wife.
Astrologically from your angle, it can be explained as under :
(1) your marital house lord (Mercury - Retrograde) is sitting with Sun (Exalted) and Saturn (Debilitated) in the 5th house and watching the 11th (Income) house. It means your In-laws are a family of influences among or as government employee who had been a source of windfall for you either at the time of marriage or thereafter or both. The Retrograde Mercury (the marriage lord) indicates your wife's thinking is of negative nature, she cannot think positively or cannot sacrifice petty selfish interests for the sake of family peace and prosperity. It is most possible that she is being encouraged by her relatives to go all out to take you away from your family so that you can be brought to their full control to suit their ends and designs. And you know it well how you can fit in their schemes.
Now astrologically, you could have been warned of this prospect had you taken some really knowledgeable people in confidence. Because alternatively, this same position does also indicate at the possibility of taking a girl from very poor family which had intellectual background without any power or influence in the society. The second prospect, if accepted, would have given you nothing at the time of marriage, but the same traits for the wife (retrograde-mercury) would have made the family a prosperous one who would have been in a suitable employment (since 10th house also belongs to Mercury - indicating a wife who will be active in more ways than one), and would have shared your responsibilities and would have got little time to think of devilish ideas of dividing the family.
(2) The decision has been entirely yours. You have opted for the first and the consequences are before you, and mind you this thing is happening in full vigour for the last one year and would remain till January 2010 because till then the trouble maker SATURN (which is placed in the worst possible combination in your chart - Debilitatedly placed with arch enemy powerful SUN in the fifth house) would be in the 7th house of your RASHI CHART (for Kumbh Rashi natives 7th house is LEO, where the SATURN is making a transit for 2.5 years, as above said, which is called ADHAIYA. In this two and half year period, which comes once every 27 years, aggravates domestic problems and causes untold miseries and hardships on the native, even if the Saturn in one's chart happens to be fairly placed. For one with poisonous Saturn, the position is even WORST which you have yourself accepted.
(3) Since in your navamansh chart, the retrograde Mercury happens to be in the 7th house watching your Lagna (ascendant), you will be seized with this type of problem so long you remain married - this wife or any other girl. So, the idea of divorce should be of no helpand at once should be discarded. There Mercury happens to be in the Venus' navamsh, which is Exalted in your main chart and is placed handsomely in the 4th house - indicating a very comfortable lifestyle through wife. Any such thinking of getting riddance would put your all money wasted on legal complicacies and harrassments for everyone in the family.
(4) last but not the least, do you know the MARS in your 2nd house is making you a MANGALIK. Because MARS in 2nd house destroys two BEST houses in the chart with a single stroke, the 5th and the 9th, and also causes negative energies to flow through the 8th house which is considered the life blood of the Native.
Since the Exalted MARS is associated with Rahu in the 2nd house, albeit at a distance of (safe) 27 degrees, its effects can nevertheless be felt by the native in full measure in his domestic duties. I hope you have got your chart matched for MANGALIK compatibility before the marriage. If not, one of the reason and source for your troubles are rooted in this factor too. And for MANGALIK- NON MANGALIK couple there can be offered no remedy, because the difference and dissatisfaction surfaces, inter alia, due to unfulfilled conjugal intimate relationships also.
Now how to overcome the hardship. As I have already told you, if you exercise patience, and tune your mind as to what are your duties and priorities, it would not take you much wisdome and boldness to tackle the problem carefully, and dilligently without offending any one. Since you have got a daughter also, you cannot come out of the marriage. You should try to convince everyone - your wife, your in-laws other trouble mongers - that you would decide about the matter only after your sisters have been married, and convince them boldly that you are taking all out efforts to get it done. Buy your time till 2010. I am sure there will come a wayout.
Do not lose faith on God almighty. Make regular prayers, and he will show you the path. But be sincere in your approach and do all your duties faithfully and sincerely without doing injustice to anyone.
Wish you all the best.