Can Their relation is strong. be accepted??
I would say no. A relationship could be good, but not strong (you don't see someone very often, e.g.).
yes, and I said that their relationship is fine, but of course it was marked incorrect.
Why can't it be his/her?
Thah would be uska not unka.
I remember the "s" in "his" to remember the "s" in "uska".
And ure the "m" in "them" to correlate with the "n" in "unka".