Is it to escape arranged marriages? Are they forced into it by their mothers? Is life as a BK Sister more attactive to them?
Yes, the arranged marriages in India bring so much family responsibilities. Now in Modern world, women also expected to do job and earn money for the family. Then the Husband-wife relationships are also breaking very often. Divorce rate is also very high in India. Just to escape all this, may be the girls (mostly from low to middle income family) opting for this lifestyle which brings love and respect for them. Also, the feeling of serving the GOD and thus humanity making them to live in the centres.
And what happens to them, are they basically used as full-time house servants after they are married off to the BK god?
100% involvement in all types of activities in the centre. Right from cleaning the centres, taking Murli class, food preparation, marketing in the public by various methods to attract new people. Every thing she has to do under the strict supervision of the Senior Sister.