It's not exactly impossible now, is it? We know Bloodmare, mother of Blood-Eyes (Ed's late mate) is Grim Eyes' grandmother, as is Bloodtail, Bloodmare's daughter, her aunt. We know (by Boneclaw Mother's words) that Bloodtail was envious of Blood-Eyes' mate. Ed's story only mentions "sister of Blood-Eyes, mother of Blood-Eyes sister" as being crazy with grief, with no mention of more family...