Unfortunate to hear of loss of natural father coupled with early separation when you were 4 yrs old and seems u r now in your early 20's. Since mother is shown to be remorse, may be still in cocoon stage due to her personal setbacks left with 2 female children to look after in support by alms of relatives and your older sister is settled you are feeling lef tout on love and affection coupled with belonging grounds after experiencing all these (infact you have unknowingly matured beyond your laughing / playing age) and you are an example of a self made personality by completing Eng. and getting employed as Sr. App. Specialist etc. In a way you have lead a tough life and come out of it remarkably well given to understand your mini briefs. Thank God for all the good times you have instead of relapsing.
Suggested to stay in good moral / ethical company and try to make visits to a good psychiatrist for boosting self morale and self esteem (don't feel shy of seeking psychiatrist helps, few sessions there will heel your past traumas and makes you as a person look and feel confident) or take few courses of Art of Living these are wonderful opportunities you should take on.
Concerning finding a suitable boy I think you should seek support of your married sister and common long standing family / friends are some of the the possible way outs.
However, since you are also attached to your mother it is suggested to look after her at this old age. Seeking legal help understanding and may be thinking to file case against your mother / relatives is not the ethical approach and your relatives who helped you all at one particular poi8nt of time now should not be involved in legal hassles for no fault of their owns. It is not the right approach you are being guided upon (thought wise).
Also suggested to move out of this legal forum ASAP as this forum is not right forum at this young age to seek legal help instead it will make you change track mentally (thinking wise) and even when a good compatible opportunity (prospective loving caring boy) comes along the way you will feel suspicious of even his good intentions. It is all trial and error and lady luck you have come with in this tiny planet and what is meant to happen it will happen no matter how much we try to change our destiny by chance or destiny by choice :-)
Well having a family setback (reactive depression possibilities) at an early age coupled with love and longing pangs and topping it with a self made secure (financial) life you should be looking to greener patches of your wonderful life. May be seeking overseas opportunity side by side looking after your mother may be one of the options worth pondering on.
BTW when people say "I don’t have anyone to hang on to" then they are the most loved ones actually - think inward you will find truth there. ;-)
I am sure as a fellow human being; you will find a way with a positive mental frame. Wish you all the very best..